NRA Highpower limits zoom to 4X at 600 years, and PRS has a military category that limits zoom to 6x at 1,000 yards, and plenty of people are hitting the targets.
More zoom does not equal more precision. Precision is a factor of the rifle and the ammo.
More zoom does let you see hits and misses better.
If you're not runnin and gunnin in competition, the SWFA 6X MQ will get you there.
Snipers in Vietnam were using 4x and 10X optics and the 10X was considered to top magnification by some. More than 10X, and mirage becomes a factor, as well.
For the most part, I agree.
Precision is also a function of reticle and target subtension, in addition to rifle and ammo.
More magnification can help you see hits/misses up to a point, but FOV (especially in a dynamic situation) and mirage play a role, as well, and that limits the useful level of magnification for spotting your shots and making follow-ups.