I do think so. Given that both share the same engine, it won't make much difference for them if you start counting all the cost that you would have with adding yet another platform b/w the two (trainee formation, stock of parts...).
They are probably better keeping their old twin seat Gripen until those are too old to be deemed safe and economical. And then only order the T-7 at the best price money can buy on this program (meanwhile 300+ would have been produced for the USAF, 100 for the US Navy (perhaps!) plus all the export models).
Given the fact that Sweden is planning to make 60 (+10) of those E variants, I feel like most of those C/D aircrafts with the similar F404 engines to the T-7 will be mothballed just like how the A/B aircraft were, unless it's utilized in some way. So like how you suggest, it might be better to just continue using those D aircrafts as advanced trainers instead of buying extra T-7s. Let the F model do the LIFT role.
Btw can anyone tell me where all the surplus Gripen A/Bs are at the moment? Excluding what they have leased and so on, there are currently around 80 aircrafts, those probably of A/B variant, not in operation. Sweden thought about using C/D parts from the existing aircrafts when producing E/F aircrafts, although it ultimately never happened. Could it be that they have done that while producing C/D aircrafts in the 2000s? I feel that such case is unlikely though. Maybe they are just resting somewhere in Sweden in mothballed state.