Politically it might appear better if the T1 jets were sold to a 3rd party and the upgrade costs used to purchase additional new build ac. Baes insist that they're cheaper to build now, and it would extend OSD, provide a more coherent fleet and buy an extra few years for Baes / HMG to find new export orders before the line has to close. F35 numbers are hard to predict; the initial buy will be far smaller than the 130 jet figure, but given that the line (like the F16's) will be open for decades to come, attrition buys and a modest fleet increase could see the total tail numbers approach 100 over the next 25-30 years.
The only realistic way for the FJ world to expand would be to have a "Tier 2" capability (eg Scorpion/Gripen) to do any sustainment activity after the 'A team' have shaped the battlefield accordingly.
You are correct, assuming someone will buy them off us, plus we could spend the money from the sale on some new builds too.
the 2 tier idea is partially why I foresee us buying a minuscule number of F35s...they'll be able to pave the way through the really nasty stuff on day 1 (at least until stealth countermeasures proliferate) and then the rest of the FJ force could hoof in behind them and quaff all the information they datalink to them. It would need yet another carrier configuration change though!