Having one, maybe two ESCs go bad could be attributed to faulty units (especially if they were v1 or v2). But more than that and I'd start looking really close at the setup itself and maybe even the component choice. Heck, unless I know the specific cause of a failure, I'd be taking a hard look at the setup after one failure. The MMM is rated for 120A @ 6s, with higher current bursts. That's a lot of power. But if some other part is causing them to burn up/go bad, chances are the same thing is gonna happen no matter what you get.

If all components are fine, and it's just a matter of really high currents, then I'd say to step up to an 8s+ ESC with the proper motor kv to match. That way, you get equal or more power with less or equal current. High current is the killer; it creates more heat (losses) which can be difficult to shed with low airflow and/or high ambient temps.

Your trouble could simply be a combination of a faulty ESC or two, low airflow, and maybe overzealous gearing. Each one of those not enough to cause a failure by itself, but together...

Alright, so I had a close look at your gearing and whatnot, and it is on the high side. However, I can't see that destroying 6 MMMs. As said, the setup is used in other places with success (tho is aggressive.) The MMM is good for 2500W or so, and lots alot of power.

Something else seems wrong, either a truck issue, setup issue, or driving issue. I would def get a ET to look at the power levels. I would def turn punch control off and learn to use finger control and even radio settings. Be triple sure there is no binding, and that there is not something else wrong, like way too light of oil in the C.diff causing too much unloading. You just can't fubar that many MMMs so fast with what you are doing (as least as it looks on paper.)

One thing that is often overlooked is extreme servo draw due to improperly set epa or binding in the steering system. Remove the pinion, power up the truck and try to manually roll the truck while holding the steering wheel full lock both ways. Also set you epa so that the servo is not pushing the wheels past the travel. A faulty servo can also overdraw the MMM internal bec. I have seen this happen 1st hand. Try removing the red wire and then powering the rec/servos with a rec pack.

Well, I will try to talk you out of the Tekin system. First off, I love Tekin stuff, I used to run that for racing as I never had any love for the "other" guys. i also am looking at the new Tekin system for another project (certainly not for my Muggy). I know you say you don't want to spend the money on a HV system and 1521. If you get the Tekin system and have the same problem, you will be spending more money than doing it right the first time (hv and larger motor).

As others have said, check the rest of the rig for problems. Too light of center diff oil can cause a lot of heat in your electronics.

I was thinking about this today (again), and considering which system I am going to put in my 8T. I could pull the MMM/Neu out of my Revo, and use it. But now-a-days everyone and their mother has a MMM/Neu. One of the neat things that "we" used to have was a uniqueness that has now been lost with the spreading mainstream popularity of brushless.
So for me, getting the RX8 would only be a step towards being "unique" again - not necessarily for any performance improvements.