Teams + Planner can they replace the Sharepoint project site?

Also, Teams is really great for Project management communications thou! I didn't even touch on that. The biggest piece is the task management part thou. Sine Planner has improved alot thou, and if you do your channels right you can have a plan on each channel to manage tasks for those topics. But rolling up those tasks etc. to a big picture are difficult.. With a SharePoint list, you can have custom columns define each step, or "channel" then have a view that rolls it all up.

So I mean it really just comes down to your requirements on the task part determine which way to go. Start with Planner, if it doesn't meet your needs, expand back into SharePoint.