I think a good comparison is how long it takes for people to make games on the PlayDate. The PlayDate has a weak processor by modern standards, but it also has a lot of modern little touches and a modern SDK.

Yoyozo[1] is a recently released PlayDate game. The first toot about working on it on Mastodon[2] is from Sept. 5 and it got into the PlayDate store on Nov. 21. Elsewhere the creator said he put about a month into it. So a lone dev working on a legacy-like platform can launch a game in a month. I think Yoyozo doesn't have quite as much depth as Donkey Kong (not as many screens, no enemies), but it was made by a lone dev, so it probably all balances out. I think people are basically still capable of the same level of output, maybe a little higher, if they're willing to work on a limited platform.

[1]: https://blog.gingerbeardman.com/2023/11/21/yoyozo-how-i-made...

[2]: https://mastodon.gamedev.place/@gingerbeardman/1110136615210...