Longer Franchises (15+) can work, but there has to be a way for TOCs to mitigate risk, and in some cases to get value from investments that last longer than the franchise.
On the latter this could be DfT guarantees, or putting investment on the RAB, for example. For the former DfT either has allow more leeway in SLC and/or fares and/or it has to take part of the risk (and reward!).
Regardless of good intentions, without these two keys in place you will not get all the investment you could, and you run the risk of failed franchises. It's no good looking historically - the period from privatisation to the recession (1997-2008) is unlikely to be repeated in the near future, and any responsible bidder is not going to project high growth for 15 years.
The more I think about it, the more I come to the conclusion that First's ICWC bid was a mistake. Yes, they were obviously desperate to win it, and they probably took a pretty aggressive view on the risks, but even allowing for this I don't think their bid was sensible (perhaps they also made mistakes in their calculations - I know their financial model score was low). Even Virgin's bid was risky (riskier than First's in the medium term). It's likely that the "failed" bidders (SNCF/Keolis & Abellio) were the ones who got it right - but their lower Premiums are not what the DfT/Treasury are after.
My conclusion - unless DfT are prepared to share more of the risk, or give bidders more freedom on how they run the business, franchises will have to be shorter. That's not necessarily a problem - they just have to find (financial) methods of allowing investment.