Oil leaks have no correlation to mechanical failure, unless of course they result in all your oil draining away (crankwalk and some other issues can cause them, but I've not heard of this on subarus)
Rocker cover leaks, or cam seal leaks that early on, are usually as a result of idiotic installation. (failing to lubricate seals before installation, not installing them flush/straight, failing to adequately apply sealant to rocker cover gaskets, use of non-genuine seals or gaskets can all contribute to premature failure)
As to 'something worse' happening, I'd highly doubt it, sounds like your mechanic just wants more of your money.