No, the vast majority of existing bugs in Mach3 will not be fixed.
I believe that due to te way that Mach3 was built, with feature added on top of feature added on top of feature, that it is too complex to fix most of the existing bugs.
While I agree with you 100%, that all Mach3 bugs should have been fixed, it's just apparently not possible, or for some reason, not worth the effort.
As far as building mach4 on a solid foundation. I believe that's what they're doing. But they're not using Mach3, it's an entirely new foundation, with the goal of not ending up like Mach3. Any bug in Mach4 should be much easier to fix, so we shouldn't end up in the boat we're in today. Unfortunately, Mach4 is about 5 years in the making, and has been "coming soon" for about the last 3 years. Hopefully, it will be worth the wait.