I am all for reparability, but personally I don't want to go back to some of the ways things were done 30-40 years ago and bikes are still great for the environment compared to the alternatives. My old car which I purchased used and had for 17 years is equal in weight to about 150 bicycles!
It's true that modern cycling uses more disposable assemblies than in the past, but I've never had any problems finding parts to repair old bikes. I recently restored a 90's flat-bar mountain bike for a friend of mine. It had an integrated brake/shifter lever that I replaced with generic 3x7 shifters. Most flat-bar bikes with Shimano-compatible drive trains use Shimano index shifting (SIS) for both the front and rear derailleurs. The drive trains on bikes with higher-end drive trains are serviceable. Even when a part is not designed to be serviceable, there are often ways to service it. RJ the Bike Guy on YouTube does this a lot.
All that said, the market is headed towards single-derailleur drive trains with a large cassette on the back. It's a change in technology. It simplifies the operation and maintenance of the bike. I prefer it. So, I may upgrade by road bike with a 3x7 drivetrain to a 1x12 drivetrain at some point in the future. I am sure tech will continue to improve so there will probably other upgrades I'll want to make in the future too.
The amount of waste generated by these upgrades and part-swaps is inconsequential in the grand scheme of things.
It's true that modern cycling uses more disposable assemblies than in the past, but I've never had any problems finding parts to repair old bikes. I recently restored a 90's flat-bar mountain bike for a friend of mine. It had an integrated brake/shifter lever that I replaced with generic 3x7 shifters. Most flat-bar bikes with Shimano-compatible drive trains use Shimano index shifting (SIS) for both the front and rear derailleurs. The drive trains on bikes with higher-end drive trains are serviceable. Even when a part is not designed to be serviceable, there are often ways to service it. RJ the Bike Guy on YouTube does this a lot.
All that said, the market is headed towards single-derailleur drive trains with a large cassette on the back. It's a change in technology. It simplifies the operation and maintenance of the bike. I prefer it. So, I may upgrade by road bike with a 3x7 drivetrain to a 1x12 drivetrain at some point in the future. I am sure tech will continue to improve so there will probably other upgrades I'll want to make in the future too.
The amount of waste generated by these upgrades and part-swaps is inconsequential in the grand scheme of things.