Hardware wallets are safe, multisigs are safe. You can be safe if you put in the effort. If you don’t want to do it, that’s your call, doesn’t mean the system sucks.
Yeah difficulty is adjusted depending on how many “devices” (simply put) are mining, the target is a specific blocktime.
What’s gonna kill bitcoin is the ever decreasing issuance, but that’s not the problem of crypto nor mining for that matter, just bitcoin.
Have you skimmed over the point that I think that Ethereum (one of the few sustainably deflating cryptos) isn’t just a currency but also a base for other things that may also serve as a currency?
Also, it’s not bad for the currency itself, just for the economy around it. And again, I’m not saying Ethereum should replace the dollar.