When trains are designed to last 30 years, having new trains in the first 2-3 years and then nothing until the next franchise seems reasonable. The point which is more pertinant is that there should be an ongoing review of the service offered (timetable, capacity etc). New trains isn't everything but a review which identifies how the service can be reviewed and improved after the first five years might work.
If you go back to the original round of franchises, some were let on 15 year terms precisely because they included new train fleets - the others were let on the basis of seven years. More recently, this became shorter franchises for all, but not before Chiltern got 20 years on the basis of rolling improvements and review (although you could argue that they are now in the lean period of improvements you are concerned about).
When I was taking about new trains, I was more thinking of extra trains (which may or may not include new trains) otherwise you could see an increase in rail capacity and then overcrowding for years and years on end whilst we wait for the next franchise to start.
However if you have a long franchise you could also see mid life trains being replaced to ensure that they'll last the franchise. As an example you let a franchise for 20 years with rolling stock which is 25 years old, that stock is likely to be replaced early in the franchise (especially if doing so increases capacity). As by doing so it gives the longest period of payback on the investment by the company making the change. As there's no short franchises those trains could struggle to find a new home any time soon, especially if they are third rail units.
As such you could have trains which need more capacity but getting it as the TOC is waiting out their franchise whilst at the same time there being trains sat idle as they are too new to scrap but to old to last out the full period of the franchise they've left.
There are ways you could make it so that there was ongoing investment in rolling stock capacity to ensure that this doesn't happen. However it's something which needs to be considered before long franchises are let.