Currently when installing a new plugin, user is redirected to an almost empty page:
one less page / click to go through
That's not necessarily an advantage
I think this flow makes usability rather worse.
1.) Most users don't know difference between installing / activating a plugin and when you see it is installed most will think they are done. Many people also don't read so by showing a green notification and roughly seeing eg installed may be confusing. I would be very careful with using green color here and how the wording starts and ends if you go for a notification.
2.) The activate button may not be visible enough or not get so much attention that users actually realize there is another step to do in order to actually install the plugin and get it to work.
Sometimes having an extra page letting the user focus just on this is better than trying to remove a page. Maybe it could be instead done in a dialog or so where user can focus on that decision whether to activate plugin or not, or everything would be done via ajax in the same dialog etc.