If they do actually beat microwave towers, HFTs will be their first and most enthusiastic customers. SpaceX would not even need any specific strategy for the money to come from HFT. For biggest revenues, add steeply tiered pricing for the best latency, maybe even a bidding process.
If they tried to keep those gains in-house and did their own HFT I would not be surprised if exchanges then found ways to define rules that weaken HFT as a whole. Just a gut feeling that you need to be part of the culture surrounding the exchanges to be in that game and keep winning.
You don't even need to beat microwave. NYC-Tokyo or London-Tokyo don't have microwave links (and won't have anytime soon) so using it for these very long distances could be worth it.
If they tried to keep those gains in-house and did their own HFT I would not be surprised if exchanges then found ways to define rules that weaken HFT as a whole. Just a gut feeling that you need to be part of the culture surrounding the exchanges to be in that game and keep winning.