The more you can do, the thinner you are spread out and less you can focus on any one area.
Time is often better spent honing and refining skills you already have. So, you are seen more as an "expert" in that area, and in turn garner higher freelance rates. i.e. work smarter not harder.
With 15+yrs experience, if you aren't making a decent earning freelancing, the problem is most likely with marketing, self promotion, and client acquisition/relations/retention, not any lack of yet more niche abilities.
Remember, there's someone out there with 15+ years experience with UX, Premier Pro, 3D Max, Cinema3D... do you really think you can compete with them if you start picking it up today? Even if you aren't directly competing for those projects, if you acquire them, they'll be the "bargain basement" projects paying you the least for the most effort.
You'll never get ahead if you are always the "novice". Even if you can do everything under the sun, plus some, you won't make a decent living. Clients don't pay more for less experience. More software knowledge merely means you can take on more low paying work. That's all. More work.. less income.
Most freelancers survive by being seen as an expert at something and having deep, encompassing, experience with solving problems in a particular area, not because the breadth of their knowledge is wide and vast, while being overall very shallow. The more "expert" one is viewed as, the higher the freelance rates can be. In turn you work less while making more.
"Jack of all Trades, master of none" doesn't work for freelancing in my experience. At least not to get ahead freelancing. You can survive with that mindset, but from what I see you'll always be looking for work and always have to convince clients you're worthy of their money. Use the experience you have in a particular field work for you rather than dismissing it.
It's one thing to start from scratch and explore possible career directions. But with so much experience under your belt already, I think you're spinning your wheels if you think new software is the answer to a more dependable or greater revenue stream. The answer is to sell what you know and explain why that 15 years of experience you already have is better than someone who started with Photoshop/Illustrator a few months ago. Just as the guy with 15 years UX experience can explain why he's probably a better candidate for UX design than you are. That may or may not be actually true, but he's got the experience in convincing UX clients it is true.
Please don't mistake my sentiment... more knowledge and a wider array of possible tools is always beneficial. But they aren't always "money makers".
Can I work with PHP/MySQL? Sure.... do I market that ability? Heck no! I know I'm not the best candidate for PHP/MySQL projects and I'd never seek to acquire such projects. However, if a client needs something to work with some template employing PHP and some visual variations, I certainly understand how that works. So, I can better target deliverables to be in a more "ready" state. The additional knowledge is used to support my primary abilities, not as a standalone ability to garner new projects.
So, sure, pick up some 3D, it'll allow you to use those skill when they are needed in a more secondary manner. But don't plan on them actually gaining new work on their own for a very long time, unless you prove yourself to be some sort of rapid savant with them (which can happen for some). :).
