anonymish
There are a number of tools that will optimize svg files for size like svgo. Output straight from Illustrator, for example, has a lot room for improvement. I also usually end up hand-tweaking for things like removing the full style and doing that in my site's CSS.
What I think is really still needed are more options besides Illustrator for generating svg content. Yes, Inkscape is overall ok but is lacking in setup ease and genera speed.
There a number of programs that are so close to doing what is needed and then won't save or export an svg. Pixelmater, looking at you.
But, isn't subsetting going to result in users now caching your subset instead of a cached copy of everything? I would think that does more harm than everyone grabbing a fully cached copy once from a cdn.