I think the engine advantage is no longer valid as Ferrari allegedly have the better engine now, we all forget that after 2014 rules there have been further changes, 2017 featured the wider tyres, changes to rear and front wings, barge board areas etc, supposed to be as big a change as in 2009, Mercedes still managed to ace those rules, you also have this current year 2019 with the new front wing dimensions (although not as big a change as in 2017) and they seem to have gotten it right. I don't think its an engine thing, I think over the years the team has refined it's process and and ensured they got the best out of any package they put together, it's not 1 thing, it's everything , they refine and simulate and chase every single advantage they can get from a slot on a flap that sits on winglet that is attached to a wing (yes they are that detailed) , I remember last year when they delayed introducing engine number 2 by a race or 2 because the dimension or height was not aligned with the specification something so minuscule like like it was less than the thickness of human hair (that's unbelievable)
They also just get on with it, when they were having issues with tyres in 2017 and early 2018, they knuckled down and tried everything till they found a solution, when they had a deficit to Redbull and Ferrari in slow speed corners, they investigated and found a solution and Hamilton got that pole in Singapore.
I am willing to bet that the engine they introduce later this year will address the straight-line speed deficit to Ferrari, you can just sense it's coming, Mercedes never gets deterred by their opponents bringing upgrades early, they just follow their plan, they believe in their process and its a damn good process.
They just do everything so well, I know a lot of people talk about Ferrari having the resource to do the same but I actually think Redbull is the team that comes close and if they could match Mercedes in the engine department could really give them a run for their money, Redbull also have a well defined process for their upgrades and in season development and they can take the fight to Mercedes,
Some say the budget cap or 2021 rules might provide an opportunity for other teams to challenge, it's possible but I think Mercedes will still be up there, their silver bullet is the process they have adopted, it doesn't matter if its a budget cap or new rules, they will find a way.
1 option that is less considered that could give maybe an opportunity is more engine manufacturers coming in, for them to do this then maybe the engine rules have to be dumbed down a little to encourage them, last I heard Ferrari was the one totally against this and were threatening to withdraw, So in a way some of the teams complaining about the Merc dominance are indirectly cementing their advantage for their own selfish reasons.
If it is to make the sport more of a spectacle and to make it more interesting then, maybe the 3 car team should be considered for some of the top teams who can afford it, a situation where only the top 2 scoring drivers points are counted towards the WCC but the 3rd scoring drivers points still counts towards the WDC, the problem with this is this opens up another can of worms when talking about budget cap as the teams with 3 cars will want a budget increase for parts and personnel and travel.
Vettel /Leclerc / Max (Ferrari)
Lewis /Danny Ric / Alonso (Mercedes)
The in team battle alone is worth it, talk less of inter-team battles.