Colossus is not a nice ride experience but it's a thrilling coaster, best enjoyed with gritted teeth. I can see why people hate it, but if you go in knowing what to expect it's still a fun ride.
Some new trains are much needed in the future, the faff and lack of room seriously slows down loading and is very uncomfortable, but this was a design oversight from the start really. Would be very wise of Thorpe Park to commission new trains, even if it cost a lot to prototype them, if they want the ride to continue for decades to come.
Or would the ride have had its day in 15-20 years anyway? Since it was only really designed to break the record and shift Thorpe Park to a thrill park, which it's well known for now and could maybe use the space for a better looping coaster? It's worth a few more years though I'm sure!