The idea that one must download the entire blockchain makes maintaining a wallet very difficult

Totally false. There are plenty of wallets that allow you to connect to a decentralized network of servers hosting the blockchain.

Electrum, Multibit to name a few.

For Ethereum as well— the official wallets will let you run them in light mode which brings the chain size from approaching 100GB down to a few GB

NEM (the coin that was stolen) doesn't make any local wallet user download the chain. Instead, it uses a client server model.

https://nem.io/

As such, the wallet is super lightweight and easy for an average user.

Doesn't protect you from exchange hacks though.

maybe, but I don't recall encountering any of these years ago

They have been around for over 5 years. I used them 5 years ago..

You don't need to download the entire block chain to generate a receive address. You don't even need to download the entire thing to spend if you don't care about privacy. You can use vbuterin's python scripts to do the whole thing off public blockchain api's.

I've messed around with Bitcoin and I can't get past the fact that the top 2 most recommended wallets for macOS aren't signed by Apple and you have to verify a PGP signature manually (and the author's key is signed by a bunch of people I've never heard of so even that is a leap of faith). So I refuse to deal with amounts bigger than I assume will definitely get stolen.

eh, not all wallet require the whole blockchain. e.g electrum