I don't particularly think VSL instruments are any more cumbersome then others, in fact quite the opposite, they are quite flexible. There is nothing stopping you, for example, from putting all of your articulations in Synchron Player under one super tall column (dimension), and only need a single keyswitch for each articulation. You can reassign keyswitches and CC's to various aspects of sample playback better then just about any other sample player I can think of.
Yes, the factory defaults happen to be setup in a way with 4-5 dimensions, which means you need to send perhaps 4-5 keyswitches using those arrangements, but you can rearrange your own presets however you want and can avoid multiple keyswitches.. ViPro also you can arrange presets using only a single row of X-based keyswitches if you want.
You are making a theoretical argument but have you actually tried these things? Synchron player does not offer scrollbars on dimensions and shrinks everything down to an unreadable and unusable size as a single dimension grows.
And VI Pro only offers 12 slots per X/Y dimension, even if the controller is a CC. Sure you could make dozens of single-cell matrices and drive VI Pro via program change but that is clearly antithetical to the design. VSL's intended design takes more control messages than any other. A single-dimensional control method can be offered orthogonally - e.g. Spitfire's UACC system (although the universality of that was an overreach). XSamples libs have a 2-D keyswitch system but also offer access to all 88 patches via a single CC, right out of the box. Don't get me wrong, I appreciate VSL's flexibility otherwise and I think a lot of the challenges come from their increased depth of sampling, which I also value.
As far as the initial ask (MIDI scripting in VEP). Sure, we can talk about the shortcomings of DAWs etc and make arguments for this (we agree) but there is already an example of this feature on the market and it is available with the vast majority of VSL's competitors - the Kontakt ecosystem.
Kontakt player, like VEP, can host multiple VI instances, albeit only Kontakt ones. And right at the top of every multi there are slots for KSP scripts. People value this capability and it gets used. If you have a lib that only uses keyswitches and you want to use CCs, you can fix it. If you want to route across multiple instances you can do it, channelize, conditionally filter etc. Thus every Kontakt instrument supports MIDI input scripting but no VSL instrument does and no VSL product can add it.
So I'll change the argument - Kontakt multis offer both MIDI script slots and a built-in scripting language for them, why doesn't VEP?