It's much, much more important to ride a bike, any bike, than it is to ride your tri bike.

I have ridden nothing but a tri bike since I purchased one back in 1991, my first was a Kestrel KM40, today I ride a titanium lightspeed, so I guess it's close to 33 years on a tri bike, rode tens of thousands of training miles on the tri bike, along with many, many triathlons over the years. I have 3 bikes, two are Kestel KM40's and the one lightspeed, all with 650c wheels. I did ride a road bike when I first started out back in 1981 until 1991. I have never had any issue riding all sorts of terrain on the tri bike, used to do the death ride up near Tahoe on them for years as a training ride for IMC. Got so used to the tri bike I guess I forget what a road bike feels like today.

I was 67 this year so I don't expect I will be purchasing a new bike, road or tri as I hope these bikes last me out. I am still doing about 350 to 450 miles per month now training, no more hundred plus milers for me thankfully those days are over but I like to ride most days.

I'll say this. I spent the first half of this year training for Xterra , with a solid month and a half of that exclusively on my MTB on legit singletrack. I find now that I'm back on my tri bike doing long z2/threshold work on the road and trainer, that I can hold a higher heart rate for longer than I could prior to putting in the work on the MTB.

As others have said, riding an MTB on proper MTB terrain is almost a plyometric workout, where you have to grind enormous power to get over roots, rocks, sharp climbs, etc. A massive difference from traditional TT training. I've found its definitely helped my ability to hold top end power (vo2) and made super hard efforts easier to manage than before.

I don't think the MTB riding did anything to help my aerobic or threshold paces much, but it certainly didn't hurt them.