If the (wooden) trap-door was intact after the fire, then virtually all of the lab was intact, too.
Modulo anything Marcone may have done.
He and Nammy may have been *very* regular users of LC, in the pre-BG era.
But there were other "interested parties" during the same time-period. Given Mab's "anti-tool" training, she may have acted (having the Way in via Lea's MurderGarden) to deprive Harry of LC, even if Marcone would have loved it... maybe even
especially if Marcone would have loved it!
... Marcone probably has people in some way involved with over 90% of the construction going on. He could easily keep it more up to date than Harry. Google Maps does it in the real world and they keep making updates year round...
And Harry has the Little Folk. If he were willing to put in the time (and it still existed) I expect he could have Little Chicago back in operation in short order.
... I think the problem is it's place in the story telling, as readers we are fascinated by it, it comes up again and again on the boards. However in my opinion there are only so many times Harry can go to that well, Jim knows that. Then it simply becomes lazy story telling.
This, I think. It's really much more a meta-consideration than an in-universe one.
Does Jim
want to use LC further?
If Jim wants LC gone, it's easy to justify it being gone.
If Jim has further use for LC, it's easy to justify how it's still there.
(Unlike you, Mira, I think Jim could use it; I don't think that well is
nearly dry, yet, and Jim spent quite a while building it up, so I expect he wants more payoff from it)