Yeah I am a bit confused on the why of some of this too. Bombers and Selens already feel pretty good for the role they play imo and I see them used heavily. Bombers getting even more of a buff feels kind of obnoxious though that's my personal feelings on it and might be a far cry from how strong ladder players feel about it, but getting a reasonable "why" would probably help it make more sense.
The mex change is the opposite of what I've seen a number of people want if I'm not missing something, if it changes anything it just further entrenches and rewards hard ecoing in team games.
Just my personal feelings on it, could be way off but that's where a reasonable "why" comes in. I'd very much like a better reason for the mex changes than "making the choice between using engineers for reclaim and assist more meaningful" because like Bennis said, that sounds like a pretty bs reason. If I can afford it and have an available engineer then that engineer is going to be assisting the mex, if not then the engineer(s) will be reclaiming or doing something else useful. I don't care if it's faster now, it's purely based on if I can afford the assistance or not. Maybe that's because I'm noob, but I don't really think that would be different for better players. Perhaps we'll see some small BO changes on maps with well established meta and builds like Seton's, but it doesn't exactly help healthy gameplay in that situation.