I disagree completely with the premise. Prices don't necessarily have to increase. We have yet to reach maturity of chips optimized for inference, in addition to the regular old factor of computing work per dollar going up over time. Moore's law might be "slowing down" but we aren't at the end of the road yet. Keeping today's features running will cost less to deliver in both infrastructure and electricity in the future.
What will more likely happen is that features and functionality will keep expanding to use more processing power. But where is the limit? I say that comes when we can render at near-realtime 8k/240hz a video (or video game if you prefer) with procedurally generated world, characters, and storyline based on the users input via whatever real-world data, UI or sensors you want to use. This might even be possible now if you are a billionaire with access to million dollar server farms. Probably my imagination isn't broad enough in estimating the limit of "personal computing" but additional computing power beyond that seems pointless for any one individual.
In any case the price of an ai product will depend on the features offered and how much hardware is needed. Probably you can run a 500 billion parameter LLM on a smart watch in 2055 but if you want that power today it seems to cost about $20 a month. I doubt anyone will try to ever charge more for today's $20 featureset.
The price of this stuff will absolutely decrease, the only unknown is how companies will roll out new functionality and how specific future features fit into the pricing tiers over time.