No, there may be an inflection point in the metric "cost per standardized transistor". But this does not equal "cost per function".
There are manys way to utilize transistors more efficiently or still make them smaller while performing the same function. You have to realize that not all transistors in a chip are of minimum size due to circuit design requirements. Furthermore, only a fraction of the transistors are actually involved in performing the logic function you see from the outside. There is an increasing number of transistors spent on "housekeeping" functions such as clock distribution, power gating, bus drivers etc.
All of these can be improved by working on circuit design, improved layout software, optimzing the materials in the transistors etc. Of course this is less straightfoward than simply making the transistors smaller and therefore more expensive. Therefore we are going to see diminishing returns, but there will not be an inflexion point in the business model of the entire industriy.