I always though there is something off about those defaults.
Hopefully Google guidelines are backed by some real science.

My only concern is the impact on $container-max-widths:

• md: 900 » 787.5
• lg: 1200 » 1073.4375
• xl: 1800 » 1448.4375
• xxl: 2400 » 2004.375

With rounding down to nearest 100px we would get: 700, 1000, 1400, 2000

100 * math.floor(map.get($grid-breakpoints, 'md') * 0.009375)
// or with math.div() to make it more obvious:
// 100 * math.floor(math.div(map.get($grid-breakpoints, 'md') * 0.9375, 100))

My biggest problem with the current ones is that they exactly match common screen resolutions (especially xl and xxl). Any sort of sidebar like edge's or firefox vertical tabs bumps you down a breakpoint for no real reason.

$container-max-widths

Yeah that seems better actually. Many screen sizes will still get a wider container with that change eg. 1080p goes from 1200 to 1400px.

We could actually use container queries now, any thoughts on these questions? #18983 (comment)

sidebar like edge's or firefox vertical tabs bumps you down a breakpoint

I think that ~55px margin is going to be perfect to account for this.