Honestly ridership in York Region is going to be very low except for the commuter rush at Richmond Hill Centre. Most of the ridership would be fed in from buses or driven to the parking lot. I am pretty sure they could have built an express subway between Steeles and Highway 7 and less people would be inconvenienced than the John Tory version of the Express Scarborough subway.
The purpose of this extension is to reduce the amount of bus service on Yonge Street, which includes Viva Blue, the 99, and a few other miscellaneous GO busses. While an express extension would be okay, there are definitely some stations (namely Clark) that are super helpful to the region's overall transit masterplan.
Side note, Richmond Hill Centre is going to grow into a massive urban hub by 2030, with most of the parking spaces there planning to be redeveloped, and even Langstaff-Longbridge, the station that was purely meant for park and rides in the old "Option 1" has no replacement in "Option 3". Also, "Most of the ridership would be fed in from buses or driven to the parking lot" literally applies to basically every TTC Subway Station. The reason why despite its size the TTC has one of the highest ridership of any subway system on the continent, beating out DC and SanFran is because of our frequent bus network and how many people use it to get to the Subway, very little of it comes from density or development. Want proof? Look at Kennedy. One of the busiest stations on the network, very little development around the station, most of the traffic comes from transfer to the platitude of busses and GO.
The subway extension isn't designed to be part of the York Region transit network but instead as an convivence for York Region to get to Toronto. The TYSSE fare structure is designed for exactly that. Since Toronto taxpayers are paying up for the subway in York Region, I don't expect anything to change unless the subsidizer changes. If someone is traveling from Clark Station to Newmarket, they won't be pleased to have to pay for both TTC and YRT fares unlike VIVA today. This isn't making it better unless Queen's Park steps in.
First, don't forget that York Region is by all means a suburb of Toronto. While legally it is its own municipality separate from Toronto, in practice its basically a Toronto dorm, and as such most of the demand is to get to Toronto. As such, yes it is a convenience for York Region to get to Toronto because that's exactly what York Region is and what it exists for.
Second, if someone wants to make a trip like that, in the worst case scenario that Fare Integration isn't implemented by the time this extension opens, the 99 is always going to be an option, which is planned to have 15 minute headways when the extension opens. While the subway would be faster, it won't be that much faster, maybe a 5 minute travel time difference. Furthermore, if someone wanted to make a trip like that, in the long term they wouldn't use Viva Blue. While unfortunately Concord GO is still in its early days of planning and is unlikely to be built, Viva Orange will eventually get a branch that terminates at Clark Station, which will all the user to take the bus directly the GO station and take a GO train up to Newmarket which should be WAY faster.
Finally, the only thing Toronto taxpayers are paying for through their city taxes is the operations of the trains into York Region, not even the maintenance. The capital costs of the section within York Region is primarily being paid for by the Feds, followed by the province and finally York Region. Part of the reason why TYSSE has such lavish stations is because YR offered to pay extra for them. The reason why this subway is pushed so hard by York Region is because this setup is perfectly acceptable for the people living in York Region, and there's nothing here "to change" (well Fare Integration would be really nice, but its not an absolute necessity in this case).