I have worked in early stage, late stage startups and bigger companies as well.
"FAANG in your 20s often makes your skills deteriorate".
This is a completely untrue statement. Big tech folks are exposed to highly scalable distributed systems from the day they join the company. To release any feature, they have to be aware of standardised release guidelines, the CI/CD process, the right way to monitor and log your metrics, etc. Because the surface area of their code is so high and the possibility of affecting existing revenue is omnipresent, big tech folks have to do more work to release a similar kind of feature than what an engineer in a startup has to do.
And this happens to startups as well. Startups have rapid delivery processes in the beginning. But then they start seeing a growing trend of bugs and resiliency issues. This is where new processes get introduced and the time to deliver new features starts increasing.
Now there are few companies, regardless of startups or MNCs, who continually evaluate their processes and trim the steps which no longer make sense.
"Lifestyle creep and golden handcuffs". I agree completely with the above statement. Big tech used to pay so high with continuous stock refreshers and the valuation increases of actual shares that you can sell, that it did seem inconvenient to go anyplace else where you don't get these kind of rewards.
I know Staff and Senior Staff engineers working in India for the past decade who have accumulated RSUs worth million dollars or more. You get to see this kind of a payout in a startup only if you had joined very early and the startup is successful.
Joining late stage startups usually don't have that big of RoI since the stock doesn't grow as much as the company readies itself for an IPO. There are also cost trimming steps that the company undertakes in terms of employee count and compensation that may not make it a lucrative place to stay. When and if the company does IPO, the shares won't have the much of an upside compared to other big tech companies.