Modularity is great in many areas. The existence of modularity is what allowed personal computers to thrive into existence and become an entire industry beyond just “Would you like an Apple, or an IBM Computer?”
For commodity markets like smartphones there are too many other drivers (specifically cost, performance and size) which make modularity a bigger burden than anything it can offer.
More often than not, modularity really makes sense from a manufacturing standpoint, and much less from the consumer standpoint. Cars built on modular architectures for examples are a great way of delivering consistent quality and features across a wide range of products, but you don’t expect your end user to start pulling out the engines and putting in new ones. From a manufacturing standpoint sometimes using common architectures is also a benefit in reduced R&D costs which outweigh the additional BOM cost for each product. Spending a few dollars per product if you can save $10 million in R&D is probably a no brainer, especially in low volume industries.
Modularity on things like PC’s, SLR cameras, power tools, or a Go-Pro accessory ecosystem all make perfect sense. You know that your user has very specific and unique requirements that can be best addressed by a specific solution, and they are willing to pay for the fact that their solution will be different from someone elses. The guy who wants the best camera for surfing will live with the fact that his camera may be slightly bulkier than the one that someone uses for mountain biking.