What Uber and other companies said is that the money they spent didn't generate the expected value they were promised. If it had generated $2 for every $1 spent, then Uber would be spending with no limits.
The lack of demonstrable return on investment means there is a ceiling to the market size, and profit is capped as cheaper models and self hosted open source models limit the amount flagship models can charge.
Long way to go till market saturation though so they will grow for foreseeable, but its a quandary of how much they spend on R&D vs giving future shareholders dividends.