That's because the MLB saw its greatest attendance in 2006 and 2007. It's been downhill since then. Also, the bump in attendance due to a new ballpark (which have been ever increasing lately) never last more than a year or two.
Attendance is going down year to year. Since 2007, the only year where attendance did not decrease from the prior year is 2015 (largely due to increased attendance in KC). However, attendance is shrinking again in 2016.
Salaries are higher, true. You know what else is higher? Pretty much everything. https://www.ssa.gov/oact/cola/awidevelop.html
Is umpiring the biggest reason that MLB is fading? Nope. But it is something that could be addressed. Sticking their heads in the sand hasn't helped MLB in the past and it won't help them now.