>not passing financial success on to its loyal customers
Companies don't pass "financial success on to its loyal customers". They pass it to their shareholders or invest it in their future production and expansion.
>If they're not careful does this sort of thing open them up to anti-trust?
No. You need a monopoly for that (and it can't be a monopoly on your own brand - e.g. "only Apple makes iOS devices") it has to be on a general product category (e.g. mobile OSes). E.g. when MS faced such issues, they had over 95% dominance on operating systems in general.