Take Bytecoin right now. Their blockchain is over 9 gigs to download.
haha, you think 9 GB is big! bitcoin blockchain size is over 160 GB right now and number of full nodes are increasing!
why? because people want to use bitcoin and it has actual usage and value.
How big will it be in 20 or 30 years? (assuming its still around).
it depends on its usage. altcoins are not really used, there are usually no transactions because people only buy them on exchanges and dump them there. nobody buys them to use or store.
so in 20-30 years there won't be much of an increase. you should see how big the blocks with only 1 transaction (the coinbase tx) are and multiply that by the number of blocks per day * number of days in 20 years and you will get the minimum rise in the size.
What solutions are going to be in place for people to download and store such a massive file?
if people want to use it, then they will run a full node and store its blockchain. as i said 9 GB, 100 GB, ... are not issues and will be less of an issue in the future if the rise is not too big and out of control like ETH blockchain size rise.
Now pretend you have 20-30 different coins all with massive blockchains.
as an end user you don't need to run a full node. you can use them in pruned mode which takes up less space (if they altcoin implements it of course) or use SPV wallets.