I don't think it was just the GSL. I think Clark was assuming more of a continuation of the open gaming environment of the 3E era, as Necromancer games prided itself on the old school feel. 4E is a very specific sort of game, far more so than any previous edition of D&D. It doesn't really lend itself to alternate interpretations or the "old school" Necromancer feel, and the GSL basically disallows you to reinvent the wheel with 4E. So the combination of the GSL being restricting in a business/IP sense, 4E being contrary to the "flavor" of Necromancer, and the GSL restricting the ability to rewrite 4E failed to live up to Necromancer's expectations.
This is my position as well. The mismanaged introduction of the GSL is worthy of a lot of blame, but there also seems to be an underlying cultural shift towards more official, more codified, more fine-tuned gaming instead of the fairly free market of ideas that characterised 3.0. Looking at ENWorld or RPGNet's D&D subforum, I am not seeing a
buzz for 3rd party products that promise a different play experience, while people are still passionately debating the merits of various 4e rulebooks. Around 2001, ENWorld was awash with discussions over Sword&Sorcery's product, the pentagram on Relics&Rituals, first edition feel, Green Ronin and all that; today, this doesn't seem to be the case.
The second point is that with 4e, Wizards pretty much
massacred Necromancer's niche and split its market. If you like "1st edition feel" or "old school", it is often very hard to like 4e's supposed innovations, or even recognise the game as a legitimate D&D edition. Sure, there are lots of people who fall into this category, but I'd estimate there were at least as many who jumped ship or just quietly walked away. It is a hard game to write old school support for.
Third, what remained of Necro's niche is now efficiently served by Goodman Games (although I consider their products to be more hit and miss) and Expeditious Retreat Press. It is not easy to return to the field, especially as a hobby company that effectively hasn't published new books for years now.
All in all, I have a lot of symapthy for Clark, but they are in a bit of a predicament. I would love to see some long-promised classics (e.g.
The Sword of Air or
Treasure Maps and Lairs), and generally more low-maintenance, low-pagecount products, but I am skeptical about their viability in today's environment.