Fluid Grid vs. Bootstrap

In the beginning, the internet was ugly. Long pages of stacked images and text, bad colors, the <blink> tag...

Then people figured out <table> layouts. It was a marked improvement over the previous method. The internet changed, drastically, from a long parade of vertically stacked boredom to some semblance of "page layout".

It was far better, and it worked well, for years.

Then came the early days of Cascading Stylesheets. A wholly different way to take near-complete control over the styling of a page. It allowed far more flexibility than table-based layouts and the internet changed again. Now pages could truly look and feel very much like the printed documents from the same company on just about any desktop monitor. The internet changed drastically again.

It was far better, and it worked well, for years.

Then came mobile devices and CSS Media Queries. Now you could change the design of the page based on the user's device. Entire websites shift and reform to the will of the handheld. Another massive change to how the internet looks and feels, a truly revolutionary way to present information.

It was far better, and has worked well, for years.

Now we have these new, bleeding edge technologies: Flexbox and CSS Grid. They represent slightly better, much more confusing way to do that same thing we're already doing. This time, for the pain of relearning exactly how to do it, and what needs to be written for which browser to get it to function, we get a largely unapparent improvement to our layouts. As far as the average viewer is concerned, we could make the switch today, with 100% of websites being magically redesigned using the latest capabilities at our disposal and the collective viewership of these miraculously reformatted sites would yawn at the sameness of it all.

The jump from "basic to table", "table to css", "css to media query based css" were all far larger leaps than these latest and greatest box-model bending layout tools.

Your sites are fine with Bootstrap for the foreseeable future. Eventually, within the next year or two, Bootstrap will move to these new functions of CSS (and with it, very likely DW as well) and you'll be writing sites using the new capabilities. You likely won't even notice the switch until/unless someone tells you about it.

I don't use Bootstrap. I don't particularly care for it myself, but there's absolutely nothing wrong with using it now, or until something much more game changing comes along than Flex and Grids.

Just my $0.02.

Legacy FluidGrids were removed from DW about 7-8 years ago.  It was a flawed system to begin with and very temperamental to use.  Subsequently, Adobe gave up on it. 

Personally, I think Bootstrap is much better than FGLs ever were for many reasons. And millions of websites worldwide use Bootstrap very successfully but it's not the only responsive framework available.  Use whichever system works best for you.