Microsoft needs a partner on mobile, Apple needs a partner for AI and services.
Don't really see why they would be vehemently against this.
And along with Continuity is a key pillar of the "Apple products work better together" marketing message.
Apple makes way more money from iPhones than Macs.
[1]: https://www.apple.com/newsroom/pdfs/Q1_FY18_Data_Summary.pdf
What other kind of leverage are you talking about?
and having imessage on other platforms can work the other way: if people use it and like it, they can be more likely to switch altogether. (in a similar way, i think the ipod brought many people to the mac.)
in terms of leverage:
if you have the dominant messaging platform, you've
(1) defensively kept someone else from having the dominant platform, and using it however they wish (if e.g. google ever got its head out of its ass with messaging and made something good, that might become dominant and draw more people towards android)
(2) you've got a frickin' platform! you can start selling apps into the messenger and make $ that way. while apple has still been shy about ads (and every1 in general has been shy about ads in messengers), they have dipped their toe in the water with ads in the app store. you could certainly imagine ads in imessage. you could use that base of connections as a good starting point to grow a social network.
and who knows what else.
a sort of analoguous situation was facebook buying whatsapp for $3 billion. why did they do that? as near as i can tell, they are not making any money on it. i assume it was a defensive aquisition: something as popular and ubiquituous as whatsapp (outside the US; in the US more modest adoption) is a great starting point to make something that will take over facebook. or simply make fb less relevent, since a large part of its utility is as universal messenger.
apple could have something similar to whatsapp just for the cost of developing an imessage client for android.
People who buy Android aren't for the most part potential Apple's market. There are relatively few high end Android phones being sold the average selling price of Android phones is $100 less than the cheapest iPhone.
WhatsApp is popular because it's free, it's valuable because of its potential for ad revenue. One of the reasons that WhatsApp gained popularity outside of the US was because it was an alternative to text messages that cost money.