When they announced it, it did not support Gmail. (Or many of the other Google services)

Takeout was announced in 2011. Takeout for Gmail was announced in December 2013. Gmail has always supported POP and IMAP which are functionally equivalent to or superior to the MBOX archive format provided by Takeout, for most purposes.

When it was first announced, it was available to everyone (I didn't have it). I think now it's open to everyone.

Yes, in fact, as IMAP/POP3 access where you could download everything to your computer. I don't see why all the excitement.

I had to migrate around 100 accounts out of Gmail (free) in 2009 and it was a pain because their IMAP implementation was sub-par and so all the IMAP migration tools (imapcopy etc..) failed/halted somewhere during the process on each mailbox. At the end we had to configure accounts on various computers and manually download them (probably two bad IMAP implementation, Gmail and Outlook, worked better together than using other tools :)) Of course POP was not acceptable because of lack of labels/folders support.

If this existed at that time it would have saved us a lot of hassle...

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I've tried to solve the exact same problem. Backing up emails and easy migration.

It has; grabbed mine last week.