Replace old macbook and old(ish) iPad Air with new iPad Pro?

Recent models of iPad Pro and iPad Air, with iPadOS16, support Stage Manager.

Stage Manager provides considerable flexibility when working with multiple Apps - in particular when also using an external display (or portable monitor) with a display resolution of at least 1920x1080 (FHD), 4K being ideal. When also used with a TrackPad/Mouse and external keyboard, Stage Manager transforms an iPad from a single display to a dual monitor (extended desktop) multi-window powerhouse.

Turn Stage Manager on or off on your iPad - Apple Support

While a Mac or Windows PC can still perform some tasks that iPad doesn’t support, the gap is significantly closing. Even professional level video editing and sound production are now well within grasp of iPad Pro with both Apple and third-party App support…

Information about Apple’s own Final Cut Pro for iPad and Logic Pro can be found here:

Apple brings Final Cut Pro and Logic Pro to iPad - Apple

What is Final Cut Pro for iPad? - Apple Support

Third-party video editing is well supported by well respected titles such as LumaFusion:

https://apps.apple.com/app/lumafusion/id1062022008

In summary, with an appropriate model of iPad Pro or iPad Air, you may find that these are fully viable and flexible options for your needs. Speaking personally, as a professional User, my own iPad Pro has almost fully replaced my desktop computers - and is now my principal go-to computer of choice.

for many an ipad even pro can't fully do the same things as a normal personal computer, like if apps are not in app store then you can't use it, there still are some file system access and multitasking not as functional as a computer

but for some it may be fine, personally I develop software, can't really do that on an ipad

I agree with Rudegar.

Ipad Pro's are awesome, I have one... but I do professional video editing and have an iMac and MacBook Pro for video editing.

My iPad Pro doesn't have the capabilities to do major video editing. If you do minor editing then you may get by with just using iMovie on an iPad Pro, but if you do anything beyond minor video editing then you may want to consider either an iMac, Mac mini or MacBook Pro.

Good Luck!