The markt share argument is just a statistical thing. If you look at it at a bigger scale then Linux is actually market leader by far on PCs. PC as in Personal Computer = multi purpose computers for personal use. This includes Android devices, Chromebooks and so on compared against Windows , iOS and OSX devices.
Even if you just count sales for desktop devices and notebooks alone then the statistics will be massively skewed. No Linux user buys a new PC with pre-installed Linux. You almost always get a Windows license with a new computer, even if you don’t use it at all. But it will make a point in the statistics while the downloaded Linux ISO will not. I have several unused Windows licenses on my shelf because the target computers were all running Linux in the end.
Internet access logs are a bad source of market share statistics too. Most Linux users will have to pretend to be on Windows (with user agent switchers) because they will only see crippled internet sites when they use their real identity (eg Amazon, Ebay and several other big players). And another thing to keep in mind: Why should a Linux user regularly visit sites that deal with Windows and Mac software only (eg http://www.steinberg.net)? So statistics from these sites won’t mean a thing. If you check the statistics of Debian or Ubuntu then they will look totally different.
Apple’s popularity helped Linux a lot in this regard as most vendors now try to create class compliant devices for OSX and especially iOS. These work perfectly fine under Linux too, including extreme low latency out of the box. And many proprietary devices just work without even having to install drivers like on Windows. EG the Steinberg CC121 is just plug and play under Linux (IIRC works with Bitwig, Renoise and REAPER). But you are right. The low end is covered very well and the high end too (with RME and others). In lacks in the middle where many devices need proprietary drivers but the companies don’t have the resources to support Linux (although it is not too different to Apple’s Unix version in this regard).
However, missing VST plugins are a big problem. Not much because of missing development resources. Many of the popular Plugin frameworks already support Linux-VSTs. So it would be pretty easy for developers to port plugins from other platforms to Linux.
The main problem for VST on Linux is Steinberg’s absurd licensing system for the VST SDK. You have to register as a developer and sign a NDA before you are allowed to access it and it is strictly forbidden to redistribute the SDK files to other people. This makes it virtually impossible for the distribution maintainers to automatically build and include VST enabled software in the mainstream distributions. As a result this licensing scheme means that every single user has to register as a developer at Steinberg, download the SDK, compile the DAW from scratch, manually install the DAW and so on. Every single user, one by one. It is possible but very cumbersome. No wonder there is no market for VST plugins on Linux if the open source DAWs are not allowed to use them out of the box.
Same goes for ASIO. It would be pretty easy to write an ASIO wrapper for alsa to simplify porting but the license system is even more restrictive than VST.