It may well be worth investing in improving the infrastructure for this event, as it would certainly be paid back over the next 4 years, then you would have an airport which would finally realise it's potential.
How can you say for sure that any investment would "certainly" be paid back? I've heard words like "definitely", "certainly", "undoubtedly" etc used in connection with MSE too many times before. What's to say that Infratil wouldn't end up pouring money in for no gain at all?
BTW, what investment would be needed?? There's a long runway, cargo handling facilities etc already in place. I don't think the infrastructure is holding anything back.
Also, I think you need to look at a map. Of course there will be economic activity associated with the olympics site, but I can't see that Manston is particularly well placed to capitalise. There's a big container port just a little way downstream and three airports closer to Stratford than Manston, two of which already have public transport connections.