From what I know the 3DS Max team has been doing various updates, fixes, and improvements to the core of 3DS Max over several releases, some of this is very noticeable, other portions of this are less so (if you dig through the list of all the fixes that Changsoo Eun posts you will see a lot of the hints of this behind the scenes work in the bug fixes and improvements that aren't notable enough, or weren't fixed early enough to make a marketing campaign that is planned months ahead of release).
The problem with just "rewriting" things is that you also need to ensure that everything is backwards complaint so that it works like it did originally, not just for compatibility, but also for consistency (because wouldn't the end user hate it if a system stopped working as it should, or operated differently between one release to another). As such this is a pretty massive undertaking. If you compare this to Maya it took them years to just move their UI & code over the QT (something that is going on with each release in 3DS Max), so its understandable why this takes time. This involved work also must be balanced with the need of new features, updates, and bug fixes, so it is quite the balancing act. I too wish it could just happen over a single dev cycle, but as someone who works in the games industry I also get that these things are never as easy as it appears, nor as simple as it seams, so we need to be patient and understand that these improvements are taking place.
With that said if you have a scene that is crashing 3DS max, you should report it. I know for a fact that the Max dev team is always interested in looking at and diagnosing problems, they might not give you an answer on what is wrong, but it will certainly give them an answer in where something is behaving unexpectedly in the 3DS Max core that can be released in a patch/update (and in some rare circumstances these updates can actually be pushed back into older releases 2 or 3 years behind the dev cycle). Speaking from personal experience I have scenes that are over 3GB in size and know of people that have scenes over 30GB in size that behave well (they are obviously slow to save, but that is expected due to how quickly one can transfer data down to your HDD).
