David said they are only looking at buying (relatively small) batches of puts when uploading large files to save on number of transactions when uploading large files.
Also the number of puts per coin will vary but be more stable as time goes on when there are a large number of nodes. The effects of farmers joining and leaving will have much less an effect on Puts/token. Plus as price of storage drops then farmers will be more willing to get less per Get with the other effect of more people using the network the number of gets rises, so there will be some relative stable returns per TB of storage.
But there is no ability for anyone to transfer their PUTs balance (if it is implemented and larger than a few thousands). The SNT will only purchase the number of Puts it can at the time of buying.
But YES there is incentive to get your SNT when its lower in price so future use is cheaper (ie cheaper Puts at the time of buying them)
As to the network being affected by farmers leaving such that data is lost would be game over for the network since it is then seen as unreliable and not suitable to be used. Other than that there would be an increase in farmers as the network grows until we reach world wide adoption and then the collapse is less likely than ever. Maybe though a new network rises and takes over, who knows, nothing is certain.
EDIT: This is the pay for multiple puts at once. Its not a balance but allocating puts to specific data to be uploaded and thus prepaid but only for the actually data chunks paid for.