1. Diamond steels do lose their diamonds after awhile; also, given enough time and use, those diamonds which manage to stick get worn down and lose their effectiveness. I strongly recommend against diamond steels for anyone. At their best, they create very toothy edges which aren't worthy of good knives. Unfortunately they also have a tendency to destroy the knives they're meant to maintain.
2. 15 years of professional use is way more than you should expect from a Global. Either you're not sharpening enough and using the knife dull, or its been sharpened down to a toothpick. In either case, it's probably long past time to replace it.
There's another possibility -- depending on your sharpening angles -- you may have created a very thick edge which wedges very easily; and may be able to restore some degree of performance by thinning the knife. I can't tell without looking at it, but considering how much mileage you've put on the knife it's probably beyond saving -- even if a thick edge is part of your problem.
3. Global knives are not made from "softer metal" than Victorinox. Globals are made from Chromova 18 supposedly hardened to 58RCH, while Victorinox are made from X50CrMoV15 nominally hardened to 57-58. Knife manufacturers are notoriously optimistic about hardness numbers. The reality is that both act as though in the mid-high fifties -- more like 56 than an honest 58. If either is harder than the other, it's too close for me to call. Chef's knives from either company have a similar tendency to go out of true quickly, and each needs plenty of "steeling" on an appropriate rod.
4. Should you need to "hone" (not "sharpen") as frequently as you do? A newer, better knife along with better maintenance and sharpening practices would save you a lot of effort. But there are a lot of contingencies.
Some knives need more truing than others; some boards make for more or less truing; so do some practices and uses. If you describe what you do, how you do it, and what you do it on, we can get a much better idea of which knife (or knives) will work best for you. A lot of line and prep guys who do a lot of prep at a high level like having a lighter gyuto for 90% of their work, and something heavier, more robust and less expensive for the "heavy-duty" remainder.
Wrapping it up:
As modern knives go, you can do much better than Global for the same money (which is not inconsiderable). But you got 15 years out of yours, and that's nothing to sneer at.
If you care about sharpness, you really need to step up to better sharpening kit than a diamond steel. After reading your post, I suspect you've very seldom used a truly sharp knife. You'll like it!
Edited by boar_d_laze - 2/3/12 at 8:49pm