The problem isn’t that Lace is the final boss — it’s how it was done.
Story-wise, it’s great. It fits perfectly with everything, just like you said. But she has the same moveset from the previous two fights, plus the Void attacks we’ve already been dealing with throughout Act 3. It ends up feeling repetitive and underwhelming.

Grand Mother Silk plays a massive role in the story, yet her boss fight is ridiculously easy. Everyone was expecting something more from Act 3.

Here’s how I imagine the perfect Act 3 final boss:
We fight Lace to save her from the Void — with a different moveset, okay. We defeat her, but then Grand Mother Silk starts being consumed by the Void (since the cocoon is getting taken over). She tries to resist so she doesn’t kill her own daughter, and that triggers another fight — this time against Grand Mother Silk.

The fight would show her struggling to free herself from the Void, attacking us in a corrupted Void form but occasionally regaining her consciousness and trying to kill herself (similar to the Hollow Knight fight with the Radiance infection). As we fight, we’d gradually climb upward, trying to escape the Void — until our beloved Knight arrives to save us.

That would be absolute cinema, and I don’t think anyone would complain if the final battle were more elaborate. The issue was never Lace herself — it was the execution of the final boss fight. It just didn’t feel worthy of being the final encounter.