There won't be ads in 1-1 or groups.

There will be ads in channels that opt in (but those already have sponsored posts in some form already often, only manually inserted.)

So, some will say: how long before they change their terms again and the ui is plastered with ads?

We'll see. So far Durov has kept his words to a degree that few others have.

Edit: I also think Telegram has dropped the cryptocurrency-idea now.