I believe the main issue with new Intamin trains is that they're wider than Colossus' are currently. So for Thorpe to introduce new trains (which make a noticeable difference to the ride experience), they would need to make station alterations, lift hill alterations, new clearance testing, etc. On top of this, new style trains on track designed for old trains could significantly alter the ride experience in unpredictable ways, so it may not even make a difference.
In other words, new trains would cost a significant amount of money (we are talking literally millions, probably more than Thorpe have spent in the past 2 years combined). And there's a, albeit small, risk it may not make any difference.
Of course, that doesn't mean there's not stuff Thorpe could do. They just have to think a bit outside the box to come up with an alternative way to improve the ride experience. But sadly, they won't put the energy into that.
Frankly, the best thing for Colossus would be to tear it down and build something new. But again, ain't never going to happen...