I don't see how this is going to make professions rare. It's definitely a money sink though. The rich people can just switch to a profession, do what they want, then switch back to their main combat class. Considering the amount of people out there who've already mastered the profs, this won't change much except make a money sink and maybe increase community interaction for those who're too poor to switch classes. I highly doubt it'll make professions RARE. You've already said it's easy to make money if you're dedicated to it. Even now the vets are basically choosing a combat class, maxing out its level, then switching to another class to level after playing around with it a bit. Same with professions. Making it so you only have one class isn't going to change anything.
You could lower abuse by increasing the costs of switching classes/professions, but as it is a newbie has to vote everyday for at least a week to be able to afford a spec. Increasing the cost will just make things harder on the newbies who're forced to stick with a single class and lack the money to switch out.