There is one honest argument against .co, and it's not that it looks unprofessional. It's that some proportion of people will type .com anyway, and land somewhere that isn't you.
What the .com actually buys
- Typed traffic that would otherwise leak — the size of this depends entirely on how people find you.
- Not having to say "dot C O, no M" every single time.
- An email address people don't get wrong.
When .co is genuinely fine
If your audience arrives through links, search, an app store or a marketplace — which covers most modern products — the typed-traffic argument mostly evaporates. If your name is spoken aloud in adverts, it doesn't.
The thing worth checking first
Look at who owns the .com. A parked page is one situation. A live business in a neighboring category is another, and that one is worth walking away from regardless of which ending you buy.
And the rest
.io and .dev read as technical and are fine when your audience is technical. .ai reads as a claim about your product; make sure it's one you want to make. .app and .page are enforced HTTPS-only, which is a small, real security win.