We use transistors because they are wicked fast and efficient. But a 4090 built from metal balls and wood blocks would still be able to perform all the same calculations. Or a 4090 made by drawing X's and O's on a (really massive) piece of paper. Or one made by connecting a bunch of neurons together for that matter.
Saying cells can multiply doesn't really mean anything, unless is gives ability to access some higher form of compute that is outside the reach of Turing machines. Which it doesn't, because if it did, it would be supernatural.
Analogue computers can always implement a Turing machine to within the bounds of that noise (and that's how transistors, which are really analogue devices, get used for digital signal and information processing).
A computer that used infinite-precision real numbers would be more powerful than any Turing machine, however unlimited-precision real numbers in the physical universe are prohibited by the holographic principle and the Bekenstein bound so we can't have them.
> It's clear that both biological sentient beings and sentient being made in factories in the future will essentially be two sides of the same coin...
> why should we consider the sentient being made of cells inherently superior to its transistor-based counterpart?
The defining characteristic of life as we know it, and unique quality of Earth as compared to other planets as we have measured them is the ability to self replicate, and the presence of self replicating matter.
Supposedly a single cell created all life on earth and is therefore directly responsible for every facet of the nature of Earth today. Over hundreds of millions of years the entire atmosphere, and entire surface of the earth as well as unmeasured parts of the sub surface of earth have been completely reshaped and defined by living cells all descended from that original cell.
We're very likely able to create human level intelligence or even super human level intelligence that operates in a factory produced robot body that can survive a human level life span with some sort of maintenance comparable to that of a mechanic/surgeon but if that being needs a factory to generate parts or progeny then it is intrinsically lacking compared to self-replicating cellular life.
Self-replication is a powerful ability that is intrinsic to life and deeply related to intelligent systems. As fabulous as a 4090 may be to us at this point in time, it is incomparable to a group of entities such as a humans that can create a civilization that can make a 4090, whatever material that 4090 may be made of.
A far more profoundly fascinating object is the factory that can make robots that can operate the factory that can make more of the robots, and better. And that's what multi-cellular life really is.
Until a 4090 type device can make more of itself and other by-products like multi-cellular life, the multi-cellular life that can make more of itself and by-products like a 4090 type device is intrinsically better.
Un/fortunately (depending on who you ask), there's also a lot of automation being developed for every stage of the industrial processes from "where do we even look for the right rocks to get out of the ground?" to "here's the RAM chip you wanted to stick in your socket".
The story you suggest seems to be built on a limited understanding of the processes involved. It's pretty hard to predict the future, especially given incorrect assumptions.