Bitcoin's difficulty adjusts on-demand. Regardless of how many or few people use it, as long as some people do then it'll keep existing and sitting there clearing transactions all day long, spitting out new blocks every ten minutes like clockwork. That's the beauty of Bitcoin -- it's a program that once started never stops running, like a watch that keeps ticking.
The work in place to build L2 systems (Lightning, Web5, etc) that can derive a secure economic system from that base consensus clock will continue on just fine.
Ethereum has a good developer network. It does work well as a decentralized, distributed compute engine. The costs are also coming down with new scaling solutions.
Paying for digital products (software, digital items) is hands down far better wtih a web wallet than a credit card.
Ethereum is probably the only crypto I can imagine holding for the medium to long-term
