I believe this may be the case but Ferrari and Mercedes are against it so probably a half hearted change or even a two tier competition from 2020.
Andi.
I don't know why people think other engine manufacturers would be against a good team with a competitive car having their engines. Think about this, if as some people believe, customer can't beat constructor, then in races in which Merc are strong, Mclaren would have a chance to come behind Merc and take extra points off Ferrari and the same argument can be made for Ferrari.
The only reason Ferrari or Mercedes would really not want Mclaren to have their engines is if they fear being beaten by Mclaren... which makes the argument that customer can't beat constructor stupid, because it's always been untrue.
99% of customer teams have 1/3rd the budget of constructor teams and as such there has been little to no competition over the years, that doesn't mean customers can't make great cars. The one time that Merc have a major advantage is in the first year of a new regulation period as they'll have slightly better idea of size/design direction. Smaller teams struggle to match their body work to small engine changes because changes cost money and on a tight budget smaller team cars usually have a bigger margin of error built in to start with. Mclaren have the money to turn around more bodywork changes and do it quicker, they simply have a bigger team in every area meaning they have more people in design, more people in production. Mclaren given the same engine are more than capable of beating Merc if they can make a good enough car. I don't think they'd have any chance next year, they'll have to make changes for a different engine, have a learning curve in terms of electronics, mapping and optimisation and it will take time to improve upon their first designs with a different engine in there.
I think they'd instantly go from struggling to get into the points to competing for top 5 though, it's a no brainer for them and Merc shouldn't fear anything from Mclaren up to 2020 and so they don't need to give them a deal past 2019 for now anyway. There is a chance Mclaren could take points off Ferrari/RBR which will help Merc.
I genuinely don't believe now that Honda have a single hope in hell of making a genuinely competitive engine before 2020 now, there is absolutely no point staying with them outside of financial issues. There is no reason for Merc not to give them a deal to 2020, there is no reason Mclaren can't get an alternative engine for 2 years and even go back to Honda for 2020 if Honda show good progress on whatever engine that ends up being.