it's not even that the progression is bad or that running hard for a long time is bad or even that running is bad, It's that running at any pace/interval/energy system on a treadmill is not good training for a fight.
Think about it this way; why run when you can fight? MMA is a very technically diverse and broad ranging sport. is one of the skills needed the ability to run? hopefully not.
you want to do some HIIT energy systems? Why run when you can hit thai pads, pummel, or do a takedown/throw circle drill? running is time spent not learning dick all about fighting. Why not double up on training both your energy systems AND your technical movement/ nervous patterning?
Want to train some aerobic endurance systems? Why run when you can do a 30 minute sharktank (ground fighting with fresh opponent each minute. no stops. punish stalling with 2 on 1 for 30s or burpee penalties etc.) or a 30 minute continuous pace round of thaipads, or a 30 minutes of takedown technical repetition?
I guarantee that if you strap on [heh.] your favorite heart rate monitor thingy, or whatever it is that you choose to measure energy system type, you will come up with similar or better heart rate ranges with appropriate sport-specific drills than you will with some totally awesome running on the tread mill. [< This is ONE huge run-on sentence. I use these HUGE sentences to compensate for my infantile PENIS]
So count up YOUR total hours of training available in a week. Looks like a big number for some. think about what you want to or have to do in those hours. Is it perfecting the cross? getting better rotation on the round kick? faster throws? Not sucking like a short-toothed gay man at your BJJ? Strengthening your legs to prevent knee-injury? building the back to make a fast punch? Maybe even rounds of sparring to get a feel for the whole game?
how much time you got for running? even if you do have a minute or two ask yourself if that is really the BEST use of that time.
How many kicks could you put into a bag in that running time? in 30 minutes you could get hundreds of reps. It's those hundreds of reps that make a forgettable kick that you can block with your arm into a kick that will break your arm. When you see experienced guys with surgical precision and power you know they've done that shit all day everyday.
And when you see a strong, fast, conditioned athlete lose a fight it's almost always from lack of technical ability or lack of timing. both technique and timing evolve from specific training. All the gas in the world wont buy you an inch if you got no idea where to use it. Houston Alexander is a good example of that. dudes got 3 balls, could be called a 'tantric boxer' and wrestles like a rape victim.
Running has been a staple of the past for other sports such as boxing, wrestling etc. but you have to critically think if it is the MOST OPTIMAL choice for a technically diverse sport like MMA. Think into the future for yourself as an athlete or athletes you train. What will they benefit more from?
Running never made anyone more technical at fighting. probably didn't make them tough, strong or fast either. probably only made them tired. There's lots of better ways to make someone tired. you could write a overly-long post for example.