Yesterday's gossip seemed to be be focusing a lot on Alonso possibly jumping the McLaren ship, but while he is not the driver he was after the scary Hungary 2009 spring accident, I think Massa would make a very competent second Mercedes driver until the "big guns" become available from the end of next season onwards.
Is that a joke? Despite not having the speed and before 2009 not being a top driver imo, he constantly crashes due to cutting in on people. After however ridiculously long he's in F1 he was still failing to finish races this year because he cuts in on people alongside him. Honestly he's failed to learn a basic and fundamental sporting lesson all the while every single year calling out other drivers for failing to learn, calling them immature and inexperienced without a hint of irony.
Even then you have to really factor in that, Hamilton is the guy Massa sees somewhat as stealing his title, they aren't exactly friends when on the track with Massa hitting Hamilton multiple times in his time honoured turning in with a driver on the inside method. If Mercedes are less dominant next year actually finishing races will be important in the constructors which is Massa's biggest problem.
Having a quick look, 2014 Bottas had 6 podiums 32 more points and retired only once. Massa retired 3 times and was classified 12th after the contact with Perez in Canada. Perez largely gets the blame but Massa as usual managed to entirely fail to predict that another car might not do exactly as he wanted, was needlessly close and I think is at least partially to blame for that. I still think that the crash into Kimi in Silverstone was easily avoidable, had he not locked up and lost control he had over half the track to steer around him while other cars ahead and behind managed to avoid Kimi. German GP, flipped his own car by turning into Magnussen... blamed Magnussen, classic Massa frankly. Embarrassing he's still doing that and did it to Alonso this season. Australia is the only one I believe he's blameless for. Overall they were more than 32 points off 2nd in the constructors, but had he got 32 points more RBR would likely have got a similar amount less and maybe he was the difference between Williams getting 2nd and 3rd.
2015, Bottas beat him again but much closer, however he had several big points losses like Kimi hitting him in Russia which would have added 12-15 points and he also couldn't start the first race which wasn't down to him. He still retired from less races than Massa, though that year it was mostly not down to Massa.
2016, Bottas had 85 vs 53 points for Massa, again less retirements, less collisions, better overall speed and simply completely outdriven over the season by his team mate.
So to sum up, him and Hamilton aren't friends on the track, Massa spent an entire career failing to learn lessons while being a hypocrite about other drivers experience, he constantly brings home less points than he should due to multiple collisions a year that are almost all his fault.
In a no.2 or potential future winning driver in a team like Merc you want reliability, lack of causing trouble, sensible decisions being made in the first corner of the race and ability to rack up the points in a constructors fight. Massa is a failure in every category. For someone who constantly gets into turn 1 issues due to having zero concept of allowing room for other drivers... for a team starting 1-2 and turn 1 being the biggest danger of the season he might be the single worst driver Mercedes could ever sign.
Bottas is everything Massa isn't, he doesn't have obvious flaws in his driving that he's failed to improve on. Most of his retirements are NOT his fault, his races finished percentage is miles above Massa over the last three seasons, he doesn't have an issue with Hamilton(though he didn't exactly help Hamilton with his one stupid move this year with the dive bomb in Bahrain). Bottas is the age, cost(cheap), reliable driver you want as a no.2, though being in a team as stupid as Williams is, it's hard to know if he has title potential or just a solid no.2 driver to bring in the points and finishes to win a constructors title.