Don't Wait If You Don't Want to Wait at Least a Year and a Half
If you start waiting for Xbox Project Scarlett you will be waiting for at least a year and a half. Microsoft confirmed a 2020 holiday release date alongside Halo Infinite.
Count on buying a new Xbox in November 2020. If you don't want to wait that long, you shouldn't be waiting for the next Xbox yet.
During this wait you will miss out on loads of games, and even though they may be backwards compatible, you will likely want to jump into new games when it does arrive.
Instead of waiting more than a year without a current system, you are better off buying one of the current models and enjoying the games that are out now and coming over the next few years.