What happens to the folks wanting to run a full-node in 20+ years when downloading the entire blockchain takes months and months to complete?
That is assuming bootstrapping the entire blockchain will still be the norm 20 years from now. Who knows, perhaps there is a way to truncate or compress the blockchain in a trustless manner in the future. Even if there isn't, the current blockchain takes about 8 hours to fully synchronize on my computer. We're still seeing quite significant improvements in both IPCs of CPUs and HDD disk size, don't think it'll be a huge problem even without improvements.
Will pruned nodes be their only option?
Pruned nodes don't solve the problem of having to download and validate the entire blockchain. 
