I bought a Rivendell about 10 years ago and it's probably my last bike. Is a steel frame heavier than carbon? Yes, a bit, but I don't have to throw it away after a crash, it rides like a dream, and the weight difference is less than the extra "water bottles" I carry around my midsection. Most of the weight of the bike+rider (which is what you have to haul around) is the rider, not the bike, and the frame is just a fraction of the weight of the bike!
Even though new bikes are getting more and more proprietary, I don't foresee a time when I can't buy a new Shimano cassette or other replaceable parts.
You should check AliExpress for those. You might be able to find some knock-offs. AE is actually really good for things like this. The other place to check is Ebay, in case someone is selling NOS (new old stock).