This topic has been discussed before in a thread here:
There are a lot of great points made in the thread above.
My take on it...
2018 had freak weather which caused an unprecedented amount of dropouts and slow times in the small women's elite field (which had a 20-30min head start on the general masses start). Results were so slow, that multiple women from the mass start posted net times faster than top-15 women from the elite start (prize money spots). This was not an issue on the Men's side, as the men's elite field started at the same gun as the masses, but just partitioned off by 10'ish meters. This is how it always was pre-2019.
Solution: The BAA rightly agreed to award those women the same prize money they would have won had they run those times in the elite start. (granted - they had the benefit of running with packs of men to block wind, so it's not really fair, but that's another facet of the debate I won't get into).
Going forward: This was an issue that could happen again. People were mad online about the mass start women not being given the glory and recognition for finishing in the top-15 because of the separate start. They cried 'sexism', because the men didn't have this issue with their start setup.
What I (and many others) think the BAA should have done:
Option 1: Say "Hey we're sorry, but this is the way it has to be. We need to have the starts set up the way they are, because this is the best way to get the elite women ample TV coverage of their race/finish, but we promise to bite the bullet and to continue doubling out the prize money if any women from the mass start posts a net time in the top-15 like we did in this crazy weather year".
Option 2 (preferable): Increase the size of the women's elite field. Add a "sub-elite" women's field that throws an extra 50+ women that get to start 10 meters behind the pro-women's field. These are the 2:40s-2:55+/- qualifiers. They're not going to cause any issues/disruptions to the pro race. They're legitimate racers. This doesn't completely eliminate the possibility of a mass-starter beating an elite starter, but it drastically decreases the chances of that possibility.
What the BAA did: They decided to solve nothing. Instead, they simply ADDED the problem to the men's field so no one could call them sexist.
Now the men have this nonsensical separated start.
So now, in the event that this, or any future year is another freak weather year, and the elite men's field has a ton of dropouts and slow times (2018 Boston Marathon 10th place was a 2:27:50 fyi), those of us 2:18 - low2:20s mass-starters could absolutely pack up and run a slew of times that would put us in the top-15, and then we have every right to be "mad online" about this issue all over again.