My first thought was that rife with opportunity was gibberish, but it turns out to be at least as common as the ripe version I'm familiar with. I'm a Brit, and NGram seems to imply both forms are predominantly US usage, but the version I know seems commonplace enough to me. Here's the combined US/UK usage chart over the past 50 years from NGram (UK usage is so low it doesn't graph at all if I restrict to just that).
Calithumpian has unearthed an 'original?' instance in a translation made 1834, but this clearly didn't catch on for a long time. I believe this is because many would feel the same as me that rife sits uneasily with something 'positive'. Both forms seem to have started appearing more frequently in the late 1800s. Initially with a tendency for rife to be used referring to opportunities for crime and other negatives, and ripe to be associated with fields.
By the early 1900s it seems to me there are two distinct expressions doing the rounds, with any given speaker presumably massively preferring the one he's familiar with. Both expressions are somewhat flawed grammatically and/or semantically; it makes little sense to me to suggest that only one is 'valid' or 'original, and that the other is simply 'wrong'.
To me, rife with opportunity is strange - I'm more familiar This dog is rife with fleas. Apparently others feel the same (there's only one instance of ripe with fleas in the whole of Google Books).
I do not feel that usage for either word is significantly affected by what I imagine are a tiny number of speakers who don't know what either/both words mean.
One reason I personally favour ripe is that I associate it with similar expressions such as ripe for [the] taking/plucking (and even plunder, since whenever that's said it's invariably 'positive' from the speakers viewpoint).
TL;DR: Both expressions really do exist, and in my opinion can be said to be 'valid'. Rife sounds odd to me, but obviously not to many others.
EDIT: As I write, this answer has +5/-4 votes, which is probably the most "polarised" answer I've ever seen on ELU. Perhaps that's to do with the UK/US usage split mentioned above ('rife' wasn't common in America until the early 1800s, and almost all earlier British usages are negative).
Strangely, although almost all online references give "negative" example usages for rife, the only one I can easily find that explicitly mentions this negative connotation is Google...
- (esp. of something undesirable or harmful) Of common occurrence; widespread.