Since they will be continuing to honor existing redirects, do they gain anything by disallowing new redirects? If it is engineering effort they want to save, they can just stop developing new features, right?
Continuing to honor existing redirects is a relatively simple gesture of goodwill -- it's absolutely what customers expect, it's good for the web, and doesn't need a lot of ongoing effort.
Rather, it looks like they're driving people to a different product intended for a different set of usecases [1]. They want to get out of the generic, commodity web URL shortener business, and drive more of their customers towards more purposeful destination forwarding.
Crucially, the new product is unlikely to be available through a web interface (at least, without an account), so Google no longer has to deal with their link shortener being used for spam and fraud.
Rather, it looks like they're driving people to a different product intended for a different set of usecases [1]. They want to get out of the generic, commodity web URL shortener business, and drive more of their customers towards more purposeful destination forwarding.
[1] https://firebase.google.com/docs/dynamic-links/use-cases/