It would help if you actually read what people told you, because I think I quite clearly outlined the challenge of expanding the Hunter Street tunnel.
I'll expand it with this: I'm pretty sure that expanding a hypothetical 10km (ok maybe not 10 but 2km) long 50m deep tunnel would actually be cheaper and easier than what we currently have, since there you could hypothetically just plop a TBM down and have that dig whilst the existing tunnel is in service. WIth Hunter Street you're going to have to acquire all of the surface properties along the sides of the street, and build significant easements because you will have to disrupt traffic through the tunnel during construcion. There is no other way around it.
Also, calling the expansions to the Barrie and Stouffville Line "Vanity Projects" is kinda yikes. If double tracking a station to allow it to have service more often than every hour is a vanity project, I don't know what I'd call spending billions of dollars to shift a station over by 1.5km.
Its an interesting argument to look at, but its still a very arbitrary criterion, especially regarding the context of what TheHonestMaple is advocating for. There is a lot more to infrastructure construction than just "deserves" and "don't deserves", there's also feasibility and cost benefit ratios. Even ignoring the whole "Vanity Project" nonsense, why does a city like Vaughan or Markham get to have frequent electrified all day service, meanwhile Hamilton is stuck with 30m service? Well its because Metrolinx owns the Newmarket and Uxbridge Subs, and thus have the full freedom to do as they wish with the ROW, and don't have to deal CN or CP trains during service hours. You don't have to do stuff like widen an existing tunnel under a fully built up street, or try and build a flyover on extremely weak and flimsy soil, all you really need to is plop another track, and give stations a 2nd platform. You can tout all you want the Hamilton CMA having 800k residents or it being a "cultural center", but if the environment makes it too difficult to provide ultimate infrastructure, then perhaps (and maybe rightfully so) these projects aren't worth it despite the 800k population. In other words, using this as a way of framing the issue as "Metrolinx wants to ignore Hamilton" is simply wrong. Now you can make this argument in regards to other projects like the B-Line, but I have my own choice words regarding that project that I think everyone here can easily predict.
I think its worth reminding people about the current state of Scarborough Junction, where Metrolinx decided to not build it at all (unless DB tells them otherwise) despite the operational and system usability improvements it can provide. This is just something Metrolinx does, they find some way to value engineer something if they don't think the benefits of building it are good enough, even if its something like "their darling Stouffville Line" that serves their precious Markham over Hamilton.