My take here is that if iMessage is truly a Better Tool, I'd be great if it was available on any computer that fulfills the hardware requirements to participate. For example, Apple could release an iMessage app for Android (and if they priced it fairly, this would still be cheaper than an iPhone, yet still turn them a profit). Or release specs so others can do so.
Do I think they should be or should feel obligated to do so? I'm not sure. I think society is having a debate about whether modern communication networks have "public infrastructure"-type characteristics and need to be regulated more in general, and this is probably also a question on that spectrum to a degree. Let's say I wouldn't be surprised if one day we come to the conclusion that the answer is yes and a larger functionality scope for what is considered basic interoperability is important to us, but there are also some good arguments against it.
Another angle to approach it from is the spectrum of arguments pro and con of bundling/unbundling of hardware and software/services (i.e. need-iPhone-to-iMessage). Governments which run environmental certification/badging schemes for example are increasingly expanding them to apply to software and considering or enacting rules that are decidedly against bundling with HW. The reason is that it tends to cause more frequent hardware replacements to stay compatible with SW/services and therefore a larger energy/carbon footprint - unbundling allows legacy HW to keep going longer on average. A project along these lines I was tangentially involved with myself was the research to expand Germany's "Blauer Engel" label to software.
What guides my thinking here is a bit overboard and flowery again, and also personal. As a software engineer and teenager myself I cut my teeth on making and maintaining one of the better-known IRC clients at the time; this was my first project that really had other users project part of their lives into it, hours at a time. For me it was quite powerful when I realized that I knew people who had met using my chat software and later gotten married and started families. It solidified my view of the engineer as a supportive toolmaker for others to better live their lives and build civilization. The modern day equivalent to the people who conceived of and made the flint tools others would use to start the fires to tell stories around. Comm tools being broadly available and interoperable and their social effects are stuff close to my heart, and stories of folks suffering from lack of access that isn't due to real technical barriers just aren't great, even knowing the complex economics driving the situation.