Don't forget that the Lightning Network could be also used for cross-chain atomic swaps. In some cases, it still does not make any sense, but it's a feature that might be used by people who want to avoid using exchanges and escrow. Some people think that Litecoin and the Lightning Network is a perfect combination. Well, payments processors are busy implementing Bitcoin LN payments.
If you ease hurdles and open up such bridges Bitcoins built in scarcity is at risk.
Is it, though? The ability to conveniently hop to other blockchains doesn't create additional supply. It's creating extra
utility, not more coins. It wouldn't make litecoins
equal to bitcoins, it simply removes some potential middlemen between the two. They're still different coins, so there will be no impact on scarcity.