Steel itself is pretty niche. Day 1 player here and I've never been put in a spot where I've felt I *needed* a steel attacker. MM Metagross is different because MM is an absurd move backed up by absurd stats. Missing MM Metagross isn't about missing out on the best steel attacker at the moment, it's about missing out on the overall best neutral damage generalist.
As far as steel types go, there is Jirachi to be released. While gimped by a lack of a steel fast move, Doom Desire in its current state previously set the bar for stupidly powerful. Jirachi being a base 100 mythical means it'll likely be a special research, allowing everybody a shot at getting one. Failing that, Dialga - even with Iron Head - is the 2nd best steel attacker overall and we'll probably have a month to raid it.
tl;dr: For steel attackers there are other options on the horizon that won't be *as good* but likely good enough. For uber tanky generalists, if you missed out you'd better hope someone has an extra they'll trade you.