I noticed that network capacity has been flat since January of 2023, number of channels is slowly going down, and number of nodes is also flat. That probably would indicate that a sudden surge in usage would increase the fees in Lightning, no?
Well, it depends what you mean with "sudden surge". I guess what you mean is that those which have already established channels use them more - in this case, fees could go up indeed, although they're so low that I don't believe this is really a problem. But I would suppose that if really a surge had occurred, also new channels should have been created either from new users themselves or from Lightning service providers to onboard new users.
I guess the flatness and even decrease of Lightning's capacity could be related to the vulnerabilities discovered last year (there was even a thread called
"The end of Lightning Network?" here due to the Replacement Cycling Attack) which could have lowered enthusiasm as they showed that LN is still not as secure as some already thought and not ready to be used for larger payments. It seems such an attack has not been observed in the wild, but the higher the payments in LN are, the higher are the incentives to try these attacks.
For its original intended use, micropayments and small payments up to the
BTC equivalent of $100-200 for example, the LN should be fine even now because it should not be viable to realize this attack for such low amounts. Thus it's really a bit strange that it didn't recover during the Ordinals wave.
Median base fee according to 1ml is currently 0.939278 sat and the median fee rate 0.000085 sat/sat. It's a bit a pity 1ml doesn't show charts for these numbers, it would be really interesting to see this indicator's evolution.
Glassnode has that metric but they are a paid option.