You are quite right NSL could potentially still fund the current pension if its customers are willing to bear the extra cost. The amount of profit on each airport contract is commercially sensitive and not published so we don't know which, if any, airports could support that extra cost. It is unlikely that many of the current contracts could and equally unlikely that many of the airports would be happy to negotiate new contracts with that kind of rise in cost. Certainly NATS management don't think they would if you look at the yellow posts in the NATSNET discussion.
NSL may be sold off or it may not. This particular vote won't make much difference to that outcome as the "new" pension arrangements will still be viewed as quite expensive by many potential suitors.