Just because you choose to allow one prestige class does not mean you have to allow all prestige classes. You want a sha'ir and it's a prestige class and it fits your world? Allow it.
You don't want the Rune Master? Don't allow it.
You want a feat that is a simplistic mimic of the Rune Master? Make it. See Magic Initiate and Ritual whatever for a model.
An optional system does not change the options for those not using the optional system.
Congrats for that theoretical exercise.
But in reality, all of what you just said boils down to "I don't like options I don't like".
Newsflash: ALL additions to the game work this way. Thus, you may select one out of the following two options:
1) If WotC listens to me, I will never get anything at all.
2) My arguments are not specific to the changes I want to paint in a bad light, and are therefore irrelevant.
Have a nice day,
Zapp
I repeat, since you both ignored it earlier:
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.
You can add to that list the list of objections you guys were responding to that [MENTION=6677017]Sword of Spirit[/MENTION] mentioned, being that adding these options locks concepts and features into that new option, and confuses categories. These are all legitimate complaints about new options. Hand waiving these objections isn't really a good response in my opinion.