I just wanted to add that if you did decide to open source MTE then I would be interested in contributing, as and when I could, as I'm sure many others would. In its current form MTE is mostly production ready (right?) and would probably only require the occasional patch were a Corona update (public release) to break it or a bug were to be discovered. It would certainly take the burden from you if we could contribute fixes. Also, at 17,400 lines of code MTE is a bit of a beast so would definitely benefit from being more modular if it were to be maintained by multiple developers. This would also facilitate the contribution and integration of new features by the community over time. For example, I use Jumper with MTE to provide pathfinding - https://github.com/Yonaba/Jumper - and would be happy to contribute the integration code as an MTE plugin, although I'd need to make that more modular myself first.
Modular Business Model
It might also be worth considering stripping MTE back and have a business model that's similiar to Corona SDK itself. With more modular code, for example, you could release "MTE Starter" for $25 and this would provide the basic core features / everything you need for a 2D top-down tile game. You could then sell modular extensions, which have "MTE Starter" as a dependency. So, you could sell the platform game module for an additional $10, say, and the isometric game add-on module for a further $10. If maintaining the physics engine aspects is more time consuming and complex then separate that out and charge more for that too, say $5 extra.
So, if a new customer wanted to make a basic 2D tile game then MTE would cost them just $25. The lower price might encourage more sales. If they then wanted to make a platformer down the road it'd be an additional $10 for that add-on (the cost is now equal to MTE's current $35 price tag). A platformer with physics? It's an extra $5 for that module, etc.
Making MTE more modular and encapsulated would not only ease the burden of maintenance and the inclusion of new features but you could also use it to increase sales and ultimately revenue. Rather than trying to be everything to everyone, as it currently is, you give developers the option to buy just the modules they need and buy additional components (such as the recent appendMap) or full extensions (isometric gameplay) when needed. It's definitely worth considering - if MTE were mine it's what I would do.
I think there's a lot of future left in MTE and the potential for some great games to be developed relatively quickly with it (quicker than would be without it). It would be a real shame for everyone - you, us, and Corona Labs - if it were to disappear forever.
Please keep up the good work and best of luck in your job hunting / new job. If we can help with MTE in any way please let us know.