I've worked on multiple products with 1 million+ users running on stock Rails MVC backends. Everything scales just fine. Until you hit a DB hardware limit you can just keep adding more web instances behind your load balancer. And DB Hardware limits these days are astronomical.

If you're actually hitting the limit of scaling our web backends horizontally and you don't have the money to deal with the problem, you might need to take a hard look at your product.

And I'm confident this is true for whatever your backend is: Django, Rails, Laravel, whatever.