Even if BTC only went to 100k.. haha.. it would still require sub-satoshi payments if you were counting every light switch on\off, or every single byte of data sent from a certain server.
Nobody's going to be counting that kind of IoT stuff on the blockchain, there just isn't enough space (bytes are too valuable).
Since I DO see that happening in the next 10 years, and it could take 5 years to pull off a fork (IF we can even pull another one off successfully, since I have a feeling it's only going to get harder the larger Bitcoin gets ), then I think the next fork is probably the one to aim for.
*shrug - I'm not saying we'll never need it but I think it won't be difficult.
I know it takes more power to compute and store.. but I would still add 8 bytes. Orders of magnitude more than 4. Then that would be it. Honest miners could limit the minimum spend. But I'd also be up for a variable precision solution, if someone had a good one.
That kind of sub-division can be done off-chain and makes sense to do off-chain, anyway (i.e. micro-payment channels). Obviously, you can't enforce a sub-satoshi payment on-chain but then a satoshi is very small, so you might be able to set up your micro-payments where you "pre-pay" the next satoshi, and then count down your usage with sub-satoshi resolution (e.g. audio-streaming by the byte or something).
Can it be done as a soft-fork ?
Depends on how you define soft-fork. It can be done without any change to the block itself, so it only requires a client upgrade to process a new tx format. This has happened already many times.