Okay, here's what I found after doing some research: Stroll has failed to advance past Q1 all season. Perez has made it into Q3 3 times. Perez beats Stroll's best time in quali by an average of .606 seconds. The only times STR came within a tenth of PER were Monaco and Austria.
The only other time he was within half a second was in Spain. Stroll's race craft has certainly improved, given he's finished better than he start in 8 of the 9 races so far this season, but he has only scored points twice and has a lower average finish than Perez.
While he certainly has improved since his horrendous debut in 2017, he hasn't done much to merit keeping his seat. He couldn't beat Massa in 2017, 2018 was a wash b/c Williams were so bad, and is being soundly beaten by Perez this far in 2019. I just dont see why he should stay.