Keep in mind that "best" is still a matter of opinion.

Xolox was the best Gnutella client and it is still in wide use, hence it's not quite dead yet.

That the developer has not enough money to continue it means that with improvements made to other clients Xolox will cease to be one of the best.

Hence, the death of xolox will come not directly from not having money but from the process of not-having-money > to no-longer-being-improved > to falling-behind-in-relative-quality > to no-longer-being-the-best > to death.

The ability to hire full time developers to work on a piece of software isn't a bad thing. You are more likely to get a better product that is upgraded more frequently out of it.

Anyone can claim to be the best, but what matters is if their users are happy with the product. If the users aren't happy, they won't keep using it, then the product will die. If the users are happy, then something is being done right, and the product will live (maybe even making to the "best" catagory).