I think this is a slippery slope.
We as a crew brief the "why" we would stop but now you are adding a another voice that is not briefed in this and may or may not have the skill/knowledge to make the decision. Over the years the RTO has been simplified to try and avoid unnecessary rejects in the dangerous high speed regime.
What if the ATC mark 1 eyeball sees a tyre burst at 140 kts and reacts by calling "stop" on a runway with little stopping margin.
I would like to see ATC able to call stop but only for definite issues such as runway incursion. Something like "abc123 runway incursion, stop i say again stop". That way at least the Captain has the chance of overruling if he thinks it safer.
It is a good point previously made about the different standards of ATC. For years PMI tower had the unhappy knack of giving you instructions during rollout above 100 kts. Not sure that that standard of ATC SA or prioritisation would make me happy that they can tell you to stop.