The salary cap is $102 million this season, and it usually goes up at least 5% from one year to the next. So call it $107 million in 2019-20. Wall will be at $38 million, so that's 36%. Wall's salary goes up in the remaining years of his contract, but so will the cap. 40% is an exaggeration.
The status of Wall's trade kicker is unclear. There's a brief article by Zach Lowe, which has been quoted by everyone, in which he says that a trade kicker cannot bring the player above his maximum salary (which is true), that there are funny considerations that would come into play if Wall is traded this season, before the maximum extension takes effect (which is probably also true), and that "sources say" that the Wizards could somehow end up owing Wall a whole bunch of extra money if he's traded now (for which he provides zero evidence and which is probably nonsense).
So let's assume that Wall is already maxed out for 2019-23 and that the kicker would not inflate any of those numbers.
The salary should not be a deal breaker. The Suns have been at or below the cap line for so long that some of us may forget that being over the cap used to be routine. The Suns aren't likely to be players for star free agents anyway (you can forget about Durant), so the only ways to get top talent are through the draft and by trade. If it's by trade, that means a huge salary. The Suns would still have their exceptions to play with.
If Ayton and Bridges are part of the core, their extensions will start in 2022-23, the last year of Wall's deal. That year could be ugly. It's also very far in the future. A new CBA could create ways to lessen the hit of expiring mega-salaries -- it's not like the Suns would be the only team in that position.
Before 2022-23, the Suns' salary situation would be fine. Anderson gets dumped, so the big salaries on the roster for next season would be
Wall 38.2
Booker 27.3
Warren 10.8
Ayton 9.6
Jackson 7.1
Bridges 4.2
which comes out to a bit over $97 million, still under the cap and with plenty of room to fill out the roster and even add an MLE player (or whatever that exception is called now).
I don't know enough about Wall to have an opinion on his attitude, and his injury history is a concern. But the Suns have preached "financial flexibility" since the end of the Nash era, and they have not once used that flexibility to make the team better. Instead we get self-defeating trades for competing PGs or inane signings like Ariza. Flexibility is useless if you don't seize good opportunities that present themselves.
If Wall is available for cheap, I say get him. Bender, Daniels, Canaan, Melton, MIL pick, PHO 2020 pick lottery-protected.
