It's not just a hard pill to swallow - it's more than that. The amount of casual gamers you'll be missing out on is so stupidly insane. I haven't been an avid Steam user until I picked up Awesomenauts in 2015, and I have a loooong loooong history of active gaming that starts way before that as well as a very technical background. And even now I don't get on Steam a lot. In fact, I recently bought two games in my endless procrastination boredom but never got around to play them more than once because for some dumb reason Steam games give me the feeling that I have to commit a lot of time to make up for booting my computer and launching (and updating...) Steam. Time which I simply don't (or should not) have, and this entry barrier is enough for me to realize I should be focusing on work. My iPhone, however, I can simply fire up an app and bam, I'm in that game again. Becomes a habit.
What's more, out of my 150 Facebook friends I know maybe 10 who might have Steam. But I swear to god every single one has a smartphone and at least knows Candy Crush.
My ex gf who would probably never touch PC gaming played Clash Royale, for instance. It mainly targets strategists but happens to scratch casual gamers' itches too. And why, besides clever marketing? It is SO much easier found and played as opposed to having to convince someone to boot their computer (assuming it runs Windows), install Steam, create and confirm a Steam account, hook their computer to their Steam account and finally purchase Hex Gambit. My ex gf started playing because she saw me play and got interested.
I think you looked at Space Food Truck's sales figures and went "eh, not worth it". Fair enough.
But not only is this a whole other type of game, no, also the novelty effect wore off after having been on Steam (and not blown up) for a while. Besides, most people who owned the Steam version had no incentive to get the iOS version (except for maybe playing on the go, which the game wasn't designed for originally).
Your news is very close to being a deal breaker for me since I'd have loved to play with my girlfriend who happens to only have a Mac and iOS devices. One customer less (her), one customer less inclined (me). If the upcoming news is 3DS support then she and I can play again, but then again I haven't touched mine in a year...
I don't know a lot about the Unreal Engine but I have seen games for mobile devices with Unreal splash screens. I have no way to tell how hard it is to wire up notification support and such, I also don't know how much they charge their mobile licenses extra, but it shouldn't be so cumbersome to enforce even the thought of ignoring the juiciest gaming market?
You should consider an iPad version to be one of your stretch goals. iPhone would be even greater but then the whole thing becomes a usability/UX chore which I understand very well nobody can possibly afford the time for.