I think there is a point there which the discussion so far hasn't considered. Everything has to be paid for. The ROSCO has bought the 707s, probably with money from a bank or other finance organisation which it must repay over time. It expected to receive an income from leasing the trains which would cover those outgoings, pay for refurbs at intervals, and produce a profit.
Now it won't receive the income from the 707s once the new stock for First/MTR arrives. In the worst case, though it's unlikely, the 707s would never be used again and the ROSCO would make a big loss on them. More probably they will find another home for them, though not necessarily immediately. (After all, the 442's were in store for a time after SWT stopped using them in 2007 and before they went to GatEx, and there'll be another gap, though shorter, before they re-enter service with First/MTR.) So for a period the ROSCO won't get a leasing income from the 707s, but it will have to repay the entity from which it obtained the finance, and storage costs. If as HH has stated the ROSCO has to offer a lower leasing cost to another TOC to take them on, and if it also has to pay to have them reliveried and perhaps other modifications, that means its planned income stream is affected for several years and perhaps throughout the life of the trains.
I don't suppose many tears will be shed for the ROSCO, but it will look at the situation and, in order to rebalance its financial position, will consider whether it can raise lease rentals generally for its other trains as new franchises come up. If it is able to do that, the costs to future TOCs will be higher. That in turn would lead to those TOCs seeking higher fares to cover those higher costs, and/or lower premiums to the DfT. Those things mean that passengers pay a bit more to travel, and/or that government income is lower, so as passengers or taxpayers the public feel the outcome.
It won't of course make a massive difference to fares or government income, but the decision not to keep the 707s will have the kind of effects I've described.
Maybe, with many other EMUs being replaced and needing new homes, the ROSCO won't be able to increase its charges generally - even though the majority of the EMUs soon to be displaced are at least 20 years older than the 707s. The ROSCO may have to face a long-term squeeze on its profits. In that case the ramifications feed through to investments such as pension funds, or to an increase in the rates at which those institutions that provide money for rolling stock purchases are prepared to lend it.
Things are inter-related, in ways that we may not normally have a reason to think about, and I think that although the decision not to keep the 707s may make financial sense for First/MTR in 2017, it will continue to produce effects for very much longer.