It seems to me there are two concepts vying for dominance on the lightning network, and it seems to me that we can only have one or the other.
Namely Zero Fee Routing vs HODL invoices.
Zero Fee Routing cannot allow for many pending HTLCs because a force close with many pending HTLCs creates a large on-chain transaction that is expensive.
But HODL invoices require many pending HTLCs. So unless something changes, one or the other is going to win. But i could be wrong.