Side chains, like Rootstock and Liquid, etc. are merge mined by BTC miners, and so they get the my fees. Likewyse, L2s like Lightning Network, Ark, RGB, etc. are settled in L1, so miners also get their mining fees.
As the halvings happen and the subsidies go down, fees will go up, replacing the former, and difficulty and therefore security will keep raising.
Sidechains, drivechains, L2s, and probably others will also come to Litecoin, eventually.
https://fountain.fm/episode/gVUvP7sTSozas9slaAuG