With the sheer ubiquitousness of class 150 and it's career, it's a wonder why nobody has presented one to a train manufacturer and just said "come up with a modern version of that"...

I’ve thought this about many trains recently. We have perfectly good ride quality on lots of trains built in the 90s (16x, 17x, 180, and best of all 22x) yet we can’t seem to recreate it now with multiple manufacturers producing dubious quality products. If it worked then just do it again but tweak to make it better!
As for 150s, maybe make them a bit quieter and get rid of the rattly windows for air con, and of course 2+2 seating. But yes, essentially it works, just refine and there’s no need to start from scratch.
There you go, my over simplified amateur viewpoint!
To be fair, as a passenger I don’t really care about the power rating of the engine or what the traction equipment is, I just want to run smoothly over some pretty shoddy old track in places, be efficiently and have comfortable interior. The latter was not a strong point of the 150!
I’ve traveled on them from Penzance to Preston and York to Yetminster (not all in one trip obviously) and they have been real work horses. Was always pleased to see one turn up instead of a bench seat on the 142/143. I just think they’re too noisy now and need a 170 style interior to make them great again.
Like them or not they’re here for another 10 years probably!