There seems to be an assumption that opening a brand to a wider audience will eventually lead to a degradation of quality. Or are you saying that offering RRL on the RL website somehow makes it of the same quality and status level of the diffusion lines?
Either way this seems to be very unsound logic, with the best counterpoint being that you can currently buy Purple Label and Black Label on the site. Your logic, if I understand it correctly, states these brands would have seen a downshift in quality when added to the site and/or introduced to a larger audience. I think everyone here knows this is not the case.
I will admit that PRL has some dubious undertakings, and that RL as a company has had its missteps (See: Black Label Denim). However, when dealing with brands that have a quality reputation attached to them (RLPL and RLBL for example), the company historically strives to retain its legacy of quality.
Now I will say that this new sales frontier might make the brand less exclusive. However, I think it is quite misguided to buy products based on their exclusivity. Just because a product is exclusive doesn't mean it's good. I think you are really buying into the designer mindset that expensive=exclusive=good or exclusive=expensive=good. Neither of these mantras are generally accurate or informed.
Since there is no evidence that a shift to a wider sales network, or inclusion in e-commerce, impacts quality of goods negatively with other RL family lines, especially those of higher quality (in fact a greater amount of exposure actually increased the quality of Black Label garments some years ago), I don't think you can make the prediction this will cause a decrease in quality.