IBM is still around. Look at its stock price. If anything, it's an example of how post-computer companies are much more nimble than their predecessors like the RCA that couldn't pivot from a dying market.

So let's invert your argument and try to find counterexamples.

Google is already close to 30 years old. It's not a new company. Microsoft is more than 50 years old! Do you have examples of large pre-computer-era companies that were at least this old and yet were able to successfully enter and expand in new markets? And not just by buying a stake in some company but incorporating new products into the parent company.

My fear is that AI will make that even more scalable, enabling even larger companies.