There's little question that Mercedes did the best job of developing its "power unit" to meet the new regulations that took effect at the beginning of this past season. That's how the Mercedes team won all but three of the grands prix this season and finished with at least one car on the podium at every single race. It's also a big part of how the teams that bought their engines from Mercedes this season managed to consistently outperform the other non-works-supported teams.
That clear advantage is why Red Bull, Ferrari and now McLaren are calling for engine development to be unfrozen. Their argument is that, under the current locked-down status quo, their engine suppliers (Renault, Ferrari and Honda, respectively) cannot possibly catch up. So unless the FIA and Formula One Management want the next few seasons to be the kind of absolute blow-outs that this past season was, these leading teams argue, the powers that be are going to have to make some changes.
For its part, Mercedes naturally counters that unfreezing engine development would send costs spiraling out of control. But then of course it stands to lose the most by re-opening engine development. If those three teams, however, closely intertwined as they are with the three other engine suppliers participating in next year's championship, manage to solicit enough support from the other customer teams and bring the matter to a vote, Mercedes may very well find itself out-numbered.