The miners will continue earning money from fees. They currently earn a 'little extra' from these fees each block.
It doesn't seem like anyone is going to want to provide resources in 50 years...
You can't know this. There will always be someone who will want to provide resources.
I understand that as miner profit is driven down, miners can take resources off-line, and mining difficulty can scale way back
Mining is unprofitable -> some miners leave -> difficulty drops -> mining is profitable -> more miners join -> difficulty goes up -> mining is not unprofitable. That's kind of a summarized version of what happens. We've seen the hashrate drop in the past.