I am not claiming to have all the answers, or that this is an easy (or a single) problem to solve. And I reserve the right to be completely wrong :)
Absolutely, as a mobile subscriber with unlimited data who (more or less) trusts my mobile provider, it is also my preference that devices behave as they do today (except when I do interact with 'legitimate' portals, they are often broken or extra annoying because of the browser selection, I sometimes turn off WiFi). I think there are other use-cases and other users have different priorities, trust in network providers, and data plans.
What I think you are illustrating is that it is technically VERY challenging to implement "make before break" for an entire network for all connections in environments that only provide limited access to a limited number of connections (and that it is particularly hard to do this in 'hostile' public access networks where there are commercial incentives for trickery). There are very different cost, benefit, and trust considerations for users. Hopefully, new protocols like QUIC and Capport can make that hand-off smoother, robust, and secure while also allowing users access to resources in public access environments (with or without captive portal and walled garden).
When I talk about 'public access', I include all Open SSIDs with and without portal, and even some wired environments. As a venue/provider, those are my option when offering public access: a) Wide Open SSID, no portal, few customer complains (and those all get directed to vendors, UE , AP, or ISP), maybe legal issues, probably have squatters hogging bandwidth, b) Open SSID with captive portal, more complaints (which is an ongoing issue that can't as easily dumped onto vendors), but also making money and have branding and analytics to offset the costs of providing public access, fewer problems with squatters, c) do a combination of things, including wired. I talk about 'public access industry' in terms of NASDAQ and private companies that represent an ecosystem where commercial marketing often centers on making it Easy and Secure to get public access around the world...
When I talk about the difference in behavior UEs have for 'public access' and 'captive portal', I think some pockets of engineers, who have responsibility for the security of their specific users, understand this difference in that you want to protect the user just as much on Open WiFi networks irrespective of the existence of a captive portal (i.e. any public access). I think some UE engineers also feel uncomfortable about supporting Open WiFi at all (and therefore give users warnings), but users demand it -- why? because of public access and ease-of-use. Today most people consider an Open SSID to indicate 'public access'. That SSID may or may not have a portal, full or partial Internet access, or all sorts of man-in-the-middle "applications". Figuring out what trickery is happening in public access networks will always be a moving target...
What I believe is that public access network providers who use captive portal feel completely justified in doing anything they could otherwise do when users connect to an Open SSID (without captive portal) public access networks. After all, the user trusts the UE, and the UE trusts the user intent to join the Open SSID public access network after it warned the user about the risks. If UE vendors want to change that behavior, I think it is as easy as enforcing the same policy for Open SSID w/o portal as with portal (i.e. a common policy for all public access). There would no longer be incentive to trick UE captive portal detection, because the public access network is being treated the same by the UE regardless. However, this will shift user complaints about public access non-usability, even for Open SSID networks with no portal, to the vendors instead of the network operator. There is a delicate balance there between what users want, what they should want, what they are willing to give up, and what services networks are trying to provide. Venues get all those usability complaints today, and the solutions they come up with are not often elegant.
As I said, not an easy, or single, problem... And one we can't completely solve (or even try to) in this WG. However, there are things we can do to improve capport nonetheless.