Have you considered that the inefficiency is a feature and provides cushy jobs for a lot of people and subcontractors? But yes, a modern telco can absolutely be run more efficiently if you operate it like a tech company and don't have to deal with decades of legacy sludge (whether bloated headcounts or heterogenous legacy infrastructure you have to support).

The problem is that this is a culture problem and once a company is ossified it is really hard to enact such change from the inside even if you wanted to because everyone enjoys the status quo (and who doesn't wouldn't be there to begin with).

Another example: have you seen the UK & EU banking scene and the boom of fintech and "neobanks" around 2017 like Revolut, Monzo, Starling, N26, etc? They managed to build from scratch on relatively shoestring budgets their own implementation of a consumer bank, something that their legacy competitors still can't replicate despite having way more budget and resources.

Unfortunately, the telco world is an oligopoly and they don't like new entrants (banking in the UK was actually a much more level playing field in comparison), so we can never actually see an experiment that proves or disproves my theory.