Privacy by default would be better indeed, but Lightning offers already pretty good privacy for the average honest user (not to protect again states spies though)
Lightning still need to be improved for sure but eventually it will be good enough, but you are right a soft fork for convenants could maybe help to build better L2s.
The dynamic block size of Monero doesn’t solve everything, at the end it still relies on a fee market to avoid spam so in case of mass usage, L2 would be needed anyway. For the moment it’s still cheap because pretty unused (1 tx every 3 sec…) but don’t believe that Monero is perfect