Rule #1 for any new design paradigm:
Requiring a DM to say 'no' to their players about using optional material is NEVER a reason not to create it.
I never want WotC to EVER be concerned with possibly making DMs have to man up and tell their players "No, you can't use this option" when deciding whether to create something new. The DM's job is to decide what will and will not be used in his or her campaign. If that means having to curtail some options that WotC has published by telling their players "No"... then that's what the DM has to do. It's not WotC's responsibility to give DMs an easy way out because they don't have the fortitude to stand up for their own beliefs.
There are some reasonable objections to adding new types of options to the game (whatever those new options are) that I think challenge the notion of "A DM can always say no".
First, DMs have no choice in accepting new options for Adventurers League. A PC is either AL legal, or it is not. And then there are groups that are private but do round-table DMing (not that uncommon) such that one DM might be fine with an option but a later DM might not be fine with it and they can't really tell a player that the PC they've been playing in that same game can no longer be that PC.
Second, you never know how a new option will turn out in the long run. DMs are not game design experts. A new option might look fine for their campaign to begin with, but turn out to be terrible in practice after a while. It might interact fine with how things are at the beginning of a campaign, and then interact very poorly with future things that are published or developments in the campaign. And it's a lot harder to take an option back once someone's been using it. The more options there are, the greater the risk of this happening over time.
Third, some of the best arguments against a new option have repeatedly been made by Paizo for their new options. That being, when they release a new option, they feel compelled as game designers to support that new option in further material they are publishing. So even if it is optional, that means it because almost not-optional over time because it gets brought up in future publish materials such as adventures and splat books. The initial optional mechanic gets expanded in a future splat book, the optional ability gets put on a challenge to the party in an adventure, etc.. And the argument Paizo makes for doing this is pretty simple - they want to support what they've previously published and feel fans of those things deserve that support. Which means DMs who don't like that option either don't use that published material (which is decrease options for the DM) or has to remove that material from the new published material (which means a lot more work, and more unintended and unforeseen consequences from removing it).
In summary, any time a whole series of new types of new options are offered there are a lot of ramifications to it (sometimes unintended) that do in fact offer a significant challenge to the idea that "The DM can always say no to a new option". Sometimes they cannot (AL or round-table DM previously approved), sometimes they didn't realize they shouldn't have allowed it in (future unintended interactions), and sometimes not allowing it means eliminated future publications from their game (because those future publications expand on and use the option they denied) which reduces the choices the DM has.