Will Amazon experience a significant brain drain due to RTO?

I doubt it, but it depends on what you mean by “significant”. Large percentages? No. Large absolute numbers? Perhaps. Significant contributors? Almost certainly.

Yes, but that's likely okay with them. Amazon's leadership are aware of the fact that some tech workers will leave due to RTO, and they're okay with that.

Sidenote: a lot of companies follow data-driven approaches to their own culture, where they try to inform policy choices using statistics. I would argue that measuring the impact of remote work on overall productivity is extremely hard, and it will likewise be difficult for them to measure how many are leaving due to RTO. Most people aren't honest on their exit surveys when quitting.

No, this hysterical reaction to what was normal only a couple years a ago will fade and business will return to normal.

I think the real question is around the distribution of performance of the people who leave as a result of RTO. If it skews towards the higher performing end, then it would greatly call into question the assertion that RTO will make the company more productive.

Amazon has had a bad reputation for employees for years. Why would a new item change that?

I’d expect this to not matter at all.

We don't know their reasons.

Ostensibly, they want RTO because it's RTO. The controversial theory is that it's all a ploy to get some portion to quit, a layoff in disguise.

The people who they actually need to keep will not be asked to return.

Depends what you mean by brain drain I suppose. People will leave. Tech workers are not very loyal anyway though so some churn is inevitable.

Amazon has enough prestige that they'll be able to recruit smart engineers to replace those leaving. I think the important question here would be if there is knowledge loss which can't easily be replaced. I'm sure there will be some, but will it be significant? Probably not.

I've seen engineering teams cut in half without significant knowledge loss. By significant I mean engineers are confused about how significant parts of the system work because the team that maintained those parts of the system are no longer around. Generally what happens is you lose people with familiarity with certain parts of the system so productivity drops temporarily, but broadly the team still understands how all the bits fit to together.