Buying SNT means that the buyer can spend it on data uploads. But, surely at some point in the future, the number of PUTs that burning 1 SNT would yield would start going down. Once this happens, I think it might continue spiralling down due to the effect I’ll describe below.
If the SNT:PUT value decreases, that means it makes more sense to hold PUTs rather than SNT. That means people could buy huge amounts of mutable data rights, and then there would be not only a price for SNT, but a separate price for mutable data. If someone wants to upload data, they then have a choice between: buying SNT, burning it for PUTs, then uploading their data; and buying rights to mutable data, then updating that data with their own.
There are good sides and bad sides to this. The bad is that it could cause some complicated economics, where SNT might be inferior to mutable data rights as a store of wealth, which would be a shame because SNT looks to be more easily transferable. The good is that the higher prevalence of mutable data could mean that junk data becomes less of a cumulative strain on the network over time.