The only reason RSVs were included in the first place was because SMs didn't have escorts at first. The RSVs were simply IN escorts with SM rules +5 pts whacked on. When the SMs were finally given actual escort models (first by FW then by SG) the rules given were better than their IN counterparts and so had to cost more, which is fine. But since they were also better than the RSVs and also had the SM rules they had to cost more than those too. This meant that the SM ships got a double bump in price, making them overcosted (Nova and Gladius at least).
So to fix this problem the RSVs were removed. This allowed the price of the SM escorts to come down, still being more expensive than an IN escort but without having a completely inferior ship for the same price around as comparison.
For example, a normal Sword costs 35 pts, an RSV Sword, with SM rules, costs 40 pts. A Gladius with +5cm costs 45 pts. That is a linear progression with the minimum price increase taken at each step. However, this leaves the Gladius costing 10 pts more than a Sword and it's clearly not worth it.
Removing the RSV version removes a step from the progression and therefore allows the cost of the Gladius to come down to 40 pts where it belongs. So for the same price of an old RSV you can buy a (superior) Gladius. Therefore the removal of RSVs allows the SM escorts to be priced appropriately. You now get a full SM escort for the price of an RSV. Hence the RSVs are superfluous. What's more, their removal is necessary to allow the appropriate pricing of SM escorts
If you really REALLY wanted to keep RSVs, for those that had painted them up, then you could just allow SMs the use of normal IN escorts (IN Ld and rules, not SM) at regular IN costs. I personally wouldn't like this. Having 2 versions of the same escort in the same fleet is ... blech. Also you'd have to disallow mixing the RSVs with the normal SM escorts, due to the leadership differential.
For those that have painted up RSVs you could always repaint them to an IN colour scheme or have them count as the appropriate SM escort or just sell them off. I would prefer any of these alternatives than just allowing essentially non-SM ships in an SM fleet.