Allowing addons does more than just not alienate people. It does SE's job for them.

UI's need to be updated along with the MMO, because regardless of how much new content you want to add to your game, it's still going to be and feel old if you don't keep the UI up to date.

When you allow addons, you crowdsource the most costly parts of updating the UI: the design, the testing, and researching what needs to be updated in the first place. All that the devs have to do is look at the most popular addons once in a while and implement them.

WOW did this, and it's why after 8 years, its UI is still the best UI available out of any MMO and an industry standard. No other MMO was able to hold the title for that long.