I have a BFA in this subject and 30+ years of experience designing graphics and interfaces back to Deluxe Paint and Photoshop 1. So I can't take an anon post seriously. What your saying demonstrates my point, it's a self-reinforcing trend that's gaining more traction, and I blame the popularity of Twitter for that. It was not common to see so many CSS grids until Twitter Bootstrap.

The problem is, non-designers are buying and selling these frameworks and themes. Often designers aren't even part of the purchase process. And how many designers would object: "You're making a mistake because you will regret the technical debt of this." Hey, more billable hours for everybody when it comes time to maintain the grid within the grid from hell, which was FUBAR code from day 1, further obfuscated by some proprietary WYSIWYG. Wow, the screenshot looks great though! How many designers even use the Bootstrap grid properly, assuming it was a good idea in the first place?

The fundamentals of composition haven't changed in hundreds of years. This is false: "Not using a grid is the sure sign of an amateur." Yes, grids go back to the Renaissance, but it's more often a crutch for a new artist copying a master, or for an OCD personality like Edward Hopper. You can feel the cold, painful rigidity of his work, it's amazing--but if the guy was my personal friend, I'd say he needs to loosen up and take off the training wheels. But hey, that's just my opinion ;-)