too much is also a problem, it is not as serious as too little but still is. if the blocks were mostly empty then it becomes easier to spam the network with more junk and cause problems. remember that spam is not just increasing fees, it has different forms. for example one form is to increase the number of UTXOs and make it extremely hard for nodes to run since they would have to load up a much bigger UTXO database in memory and look things up.
you can't compare bitcoin with altcoins either. because there is no incentive to spam attack an altcoin but there are lots of incentives from all sides to spam bitcoin!
LN CLTV locks increase UTXO's because the longer funds are locked as UTXO's and the more tx's getting locked. the more UTXO's there are stuck in ram not moving for weeks. which makes it a waste to even have them in ram.
as for spam attack bitcoin.
easy solution.. implement a fee priority.
instead of everyone bidding for top price. have a formulae where if your spending funds with less than X confirms you pay more then someone who has XXX confirms.
that way spammers that need to spend multiple times a day pay the price.. or use LN for its niche. and those that use bitcoin irregularly are not forced to use LN and not forced to pay the same price as a spammer.. win win for everyone