1. You're not going to truly release the social value of a monopoly unless you socialize it, either through social control or regulation. Without that, the monopoly will mainly serve to benefit its owners.
2. Monopoly R&D expenditure will likely be less than the excess profit captured by the monopoly's owners, and mainly motivated by potential regulation. IIRC, the main raison d'être for Bell Labs to exist as it did was to convince regulators to not break up AT&T. Without that threat it would have likely been more focused on AT&T's profit motive and less on generalized scientific advancement.
3. Monopolies (socialized or otherwise) will more likely end in stagnation.