[SippinLean]

>Why create a grid system with bootstrap,

Bootstrap is MUCH much more than a grid system

>vs just creating a grid using the CSS that all current browsers support?

Because the users that contribute to my conversions don't strictly use Chrome and Firefox? Because even if you are using CSS Grid a framework for it would still be a time-saver?

>but many of us don't have the budget to support such a small amount of devices

I don't have the budget to alienate my users using these devices.

[nailer]

Grid works in Safari and edge too. The only thing you get with old browser support these days is IE people. And sure if they're 90% of your conversions, as mentioned in the comment you're replying to, that's great and go for it.