I remember reading it, but I could be wrong. Telecom, fibertel, and personal are all the same now, and I had read that the spanish were considering getting rid of argentine movistar. I know I am switching, movistar is very spotty coverage.
The history is a bit murky, but as you say, Cablevision, Fibertel, Arnet, Nextel and Telecom, including Flow and Personal are all now the same company, by way of mergers and purchases around 2017 or so.
Movistar's owner, the Spanish company Telefonica Internacional, wants to exit Latin America. I think they were badly hit by the exchange controls in Venezuela, which was one of their biggest operations, and were no longer able to extract their profits from the Movistar network there. As far as I know they bought real estate with the Bolivares, since there was no way to get the money out of the country anymore.
Given that Telefonica plans to exit LatAm (other than Brazil, where they own the Vivo network), they haven't been investing in their networks here. Movistar in Mexico is already a virtual operator, and I understand plans are advanced for the same concept here in Argentina, so another company (Claro or Personal, and I understand Personal is the favoured choice at the moment) will run the radio infrastructure, while Movistar will exist in name only, with their shops, so consumers won't notice too much. However, their services will depend on the new host network, and it's almost certain their users will not necessarily receive the best quality of service. You're right to switch.
