Most of us get new equipment every 3 to 4 years - some earlier, some later. New watch, new head unit, chest strap.
I am interested to know what you do with your old equipment …do they go directly into the trash? …are you selling it? …do you have some sort of historical “museum shelf” where you exhibit these at home?
Me for (of course) ecological reasons I am usually trying to sell my old stuff - unless it is heavily worn up.
I tend to hoard.
But old chamois go up on ebay.
Depends on what it is. Stuff like watches and HR monitors I won’t upgrade unless my current one is broken. Head unit I’ll usually sell, even for a low price I’d rather somebody else be able to use it. GoPro had a trade-in program that I’ve used a few times that’ll give you a discount for a new product to send the old one in (I do the same with my iPhones).
For bike parts themselves, sometimes I do a hand me down system with other bikes. Like when I bought my Aerofly handlebars, I put the Enve bars on the gravel bike. New Dura-Ace cranks, Ultegra go to the trainer bike.
There’s always the Pay It Forward thread on here where you can give things away to other riders.
Use it until it dies because I can’t afford upgrades. Maybe for a couple of months after that.
Is 8 speed ultegra considered ‘retro’ yet? Asking for a friend.
I’m planning to clear out a lot of my gear closet this summer, so if I can’t sell some of the items, I’ll probably just give them away. There’s a local Tri coach who works with high school kids and I was able to hook them up with my old garmin tri watch. I’m sure I’ll find more goodies as time goes on. I’m a hoarder at heart, but it’s good to start fresh and help out a young person just getting into our sport.
I tend to only buy new kit when I’ve wore the old kit out. The exception to that was a new frame a couple of years ago. The predecessor was only 6years old and although it wasn’t worn out after 40,000 miles. I fancied something more up to date.
Are we? I usually like to keep things until they are worn out for the most part.
My mountain bike is close to 9 years old and completely worn. Nothing is broken-broken, just adding up all the wear items that need to be replace make it a non-starter for me to keep it. My plan is to give it away for free or give someone parts if they are interested in them.
Ditto for my smartphone: my iPhone 7 is almost 5 years old at this point, and the battery has gotten very weak. I think I will replace it this year, but I might snap up an Apple Watch instead and wait another year instead.
On the rare occasion when I do sell something, I tend to offer things for a good price. I am selling my previous road bike right now. I’m giving the person a good deal, also because I think I got a good deal when I bought it.
I refurbished my 6S with a new battery from ifixit for $25. Good as new. F— Apple and their $1000 phones. 
I tend to just keep old bike parts in a box forever. You never know when you might need them?
A few years ago I sold off all my old 8-9-10 speed Campagnolo stuff and I was getting prices that were close to what I paid for the stuff back in the day.
Today, I tossed an old jersey in the trash. It’s one I hate - someone gave it to me and I’ll never wear it. I do have a 22 year old Mercantone Uno pink Giro leader jersey (Pantani) in my closet. I never wear it but it looks cool hanging there.
Other than consumables (chain, cassette, etc), my new equipment in the last year or so has been
a) helmet (crash replacement so binned the old one)
b) new bike computer (upgraded from Edge 500 to 830); the old Edge went to a friend who is relatively new to cycling. The 830 I got is second hand from a cycling buddy who bought it but found the screen too small so I got it at a discount from him.
I gave away a lot of the stuff I’d been hoarding (old garmin watches) when we moved cross-country about 18 months ago.
I am thinking about replacing the battery. If the local Apple Store hadn’t closed, I would have done that already, but I am a bit lazy 
Money-wise, the iPhone is worth it to me compared to most Android phones. Most Android phones only come with two years of guaranteed upgrades, and that is typically measured from the time of introduction to market. If you stretch the ~$800 I paid (iPhone 7, 256 GB), that’s less than $200/year. This seems worth it to me.
I don’t want to start an iOS vs. Android flame war her, I’m just saying that when I make purchases, I typically aim to use these products for the lifetime of the product.
Straps and HR transmitters end up in a drawer or garage case. Never know if its just the battery or the contact points. The straps get cleaned up and retried the next time my HR is having problems. Those which don’t work with a known good transmitter get the boot. I’ve got 5 or more transmitters hanging out “just in case.”
I’ve never replaced a head unit that wasn’t broken AF, since Garmin’s refurb program was pretty good the last time I had a malfunction.
Bikes - I’ve yet to sell a bike, but I did donate my first bike shop bike (that I outgrew 15 years prior to giving it away). I’m still looking for the replacement (nostalgia style) for it. I might just sell my “away” bike that I keep at my in-laws since I’m probably going to buy a bike box, and since it is a hair too small anyway.
Bike parts go in a bin unless they’re truly worn out. I’ve got a 9 Spd tiagra/105/FSA combo that hasn’t been on a bike in years, but you never know… Well, I’m missing the broken right shifter, and now that I think about it, the RD is 10spd, so its basically another 10spd group… I should find another frame I don’t need…
Things like BBs get kept as a backup, but I don’t stockpile backups.
Bib shorts get donated very early if I don’t like them, or get trashed if damaged. Once they’re worn out, they’re trainer only until they’re not comfortable, then trash. Jerseys get worn until they’re trashed.
um just started TR a few months ago riding my steel 1984 Somec that I rode to junior/cat 2 glory as an enthusiastic if obtuse youth
I either sell (if the $$ return is worth the hassle) or give away to someone I know
I keep stuff for spare parts. I had an old reverb dropper post in need of a rebuild laying around for years and just donated it to a friend after all this time