It's the range of d20 products as a whole - with all the innovation and diversity that all the various companies have built, and all that can yet be made, the sort of thing that ENWorld has always been built around - that's useful to keep and support and develop. It's good to know that someone's going to be supporting something that you can still use the Book of Eldritch Might with, for example. It's good to know there's going to be a campaign setting where we might expect to see monsters from all the open-content bestiaries that have been published in the last seven years. It's even better to know that some of the monsters, feats, classes, and so on from all these d20 books in all their richness and creativity will be modified freely, improved and expanded upon. The d20 system can be a living, breathing, evolving entity. It's what it was intended to be, but Wizards never really took advantage of third-party innovations. It's exciting to know there's a company that can.
WotC's strategy makes sense for them. If they don't periodically make all their old stuff obsolete, they have a diminishing number of things they can sell people. Their best sellers are things like Complete Warrior, but Complete Warrior II is inevitably not going to do so well. As people have the books they need, they have less reason to buy new things. So it's in WotC's best interest to, every five years or so, come up with a new system that's so awesome that it's worth it for you to throw your earlier stuff away, with the knowledge that in another five years they'll be talking about how horrible 4e was and aren't we glad that 5e's finally coming along to save us all.
Paizo has a different business model. People always need new adventure paths, even if they're holding tightly to their copies of Races of Stone or The Book of Hallowed Might or The Assassin's Handbook and don't need new versions of those things. At the same time, they can still take this opportunity to make backwards-compatible improvements to the rules. In theory, they could come out with a new rulebook every year, updated with all the latest d20 bells and whistles, and you could use it or not, and the adventure paths would still be perfectly usable in your game. That's the ideal, anyway.
In a fully open system, other d20 companies could embrace this standard and continue to improve on it. It'll likely be a smaller market share than D&D, but it's one with real advantages. It's like the difference between Internet Explorer and Mozilla Firefox, or Windows and Linux. It's what the OGL was made for, even if WotC never knew it.