Is a 2019 i7, MacBook Pro adequate for multitasking, basic usage in 2024?

Maybe. But apps may no longer run, Safari may no longer be supported on certain websites including Youtube at that point, and you may need to find an alternative browser. It may still work, but will require more effort and time to make it work as it gets older.

You may be able to squeeze 7 or so years of use out of it. But 10 would make the computer a 15 year old device at that point.

Would you be using a 12 to 15 year old computer today for what you do?

Yes. It should work for your purposes for around 4 to 5 more years However, it will likely stop receiving major macOS upgrades to new versions in around 2 or 3. with security updates continuing for a couple more years after that.

Starting with, I think, macOS Ventura, Apple introduced a few software features in macOS that required an Apple Silicon processor. However, most were not productivity items and are not deal-breakers, at least for me. I'm still using a 2012 Macbook Pro as my travel computer and it does all I need on the road.