I don’t disagree with you, some more keywords might be helpful and who knows, maybe we will bump up the # of keywords in the future. But think about it this way for a moment: A customer will typically type 1, 2, maybe 3 keywords into the search field when they are looking for something. Let’s say they type “piano romantic”.
As long as you have “piano” and “romantic” in your keywords, you’ve done everything you can to tag your file correctly. Customers aren’t going to search on “piano romantic tuba lonely sad mournful percussion violins” to find what they are looking for. So in many ways, 15 keywords is MORE than enough to describe your music.
Simultaneously, you are forgetting that you can still incorporate important keywords into your description, no matter how good/bad you think your English is. (Your English sounds fantastic to me by the way!) 
Take a look at this:
Old way:
piano tuba lonely sad mournful pensive romantic film video
New way:
This is a romantic melody with slightly mournful undertones played almost entirely on the piano and accented with tuba. Somewhat sad and pensive, this piece of music is most suited towards film and video.
That doesn’t really take long to do and we’re not going to penalize anyone for poor English (unless it is totally incoherent). I also think this looks and sounds more professional to the customer than a list of 50+ keywords on a page.
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