E.g. most video games support stuff like this, but they definitely don't use microservices.
Pretty sure you could do it with a new view and an if statement in any MVC every built...
Sure they might? I can't see a category of business features that would naturally lend themselves to being microservices rather than extended features of a monolith unless there was some real technical challenge that was otherwise more easily solved by being in a microservice.
But as a customer, if I have a HR app from vendor A, and then I want to add a pet insurance app from vendor B, the last thing I want to do is to roll out some creaky enterprise integration process/framework/product. If those products work together via APIs why should I take any of the burden of integration?
"APIs" are an enterprise integration technique [1]. There's nothing inherently creaky or frameworkey about them.
[1] http://www.enterpriseintegrationpatterns.com/patterns/messag...
"You pick your "main" software, and customize it with third-party services" - not much difference monolith vs microservices
"None of it runs on your premises" - totaly unrelated to monolith vs microservices
"No party is willing to expose much more than an API" - again consuming an API can actually be easier in monolith vs microservices
Perhaps the early microservices evangelizers such as Netflix have created this environment - after all, who cares whether Netflix is a monolith or a swarm of microservices as long as billing works and users get content when they want?
There's another strong driver for microservices however, which is customer choice, particularly in SaaS settings.
In our industry (HR software), there is no one right answer, or one business problem - customers demand choice, every customer has slightly different needs, and microservices are indicated because they allow customers to pick and choose. The same is true for most SaaS areas I would say.
As an example, one tiny sub-area of HR software, employee engagement improves when we offer employees meaningful benefits. It turns out that many employees love their pets, and offering pet insurance could be a great perk (the HR space is so big that just this niche within a niche could be a multi-million dollar opportunity for someone with a good offering).
Yet would anyone seriously consider altering a monolithic HR application to add pet insurance management? Never. The only viable solution is a microservice, that does just this, and does it well, and plugs into the rest of the stack via APIs.
tl;dr: in the SaaS world, microservices are sometimes indicated not for architectural reasons, but to support customer choice, which is far more important.