Oh yes, Wikipedia will be a great help for the DM to add an "Abbey" to a polytheistical world where priests perform miracles on a daily basis and temples have their own private armies to combat the follower of evil gods.
What kind of influence does an Abbey give you? What income if any? Do the PCs even own the Abbey or does it become property of the church? Do they get minor divine spellcasting for free?
And instead of 80 pages of magic items, 60 would have done it too (plenty for the 5-7 items a lvl 20 party has, a point most games won't even reach) and they would have had plenty of space to have suggestions and guidelines for the things they have randomly thrown into the book. There would even have been enough space for working business rules (caravan guard is a pretty comon adventurer starting point, so why not have a paragraph about owning said caravan?) instead of making everything but farms suboptimal.
I can't speak for everyone else, but I'd much rather have an additional 20 pages of magic items than detailed rules on running an abbey. The magic item selection in the 5e DMG felt similar to the 2e DMG, which made me very happy (because even before I learned to actually play D&D, I would spend hours studying those entries).
As a DM I can easily make up what running an abbey does, in the same way that I don't need 5 pages of sewage tables to be able to tell my players whether the streets of a city are pristine or filthy. Running an abbey should largely depend on what the player wants to do with it in any case, which is not something any rule set will be able to cover. If those dwelling within spend all of their time meditating, then it can't be run as a business (unless the player comes up with a clever idea to market meditation classes). On the other hand, many abbeys had gardens, so if the player wants to tend a farm I say let them. Whether the PC or the church owns the abbey is very much campaign specific (some orders might allow clerics to own property, while others might not).
The rules already make it clear whether you can get free divine spells cast. The abbey grants you skilled and unskilled hirelings. Hirelings, by default, do not have spellcasting abilities, so if you want free divine magic you'll have to recruit that talent yourself. You could get free first aid though, if you have a healer on staff.
And of course, if the PC is devoted to the god of chance, their "abbey" might more closely resemble a gambling den than a house of prayer.
Not only do you not need rules for an abbey, no set of rules could possibly cover all of the things that a PC might want to do with it. Players can do what they want with their stronghold, within the limitations that the DM imposes. Maybe WotC will publish a 5e book of Strongholds down the line and you'll get your wish for more rules, but I don't think I'd buy it.