Yes obviously. You'd have to do research if you're going to introduce new mechanics for a 1444 start in EUV too. My point is that when you're researching the year 1444 you don't just type 1444 in a search engine and than enter. Doing research from that era encompasses sources from a larger time period - and thus the "decade of research on the time period" is not going to be wasted as they'd have to start over again as you insinuate.
Also there's the work from all the other titles ranging from Crusader Kings through Europa Universalis 1-4. Doing research is the name of the game with this genre, and they're very likely not going to start over from picking a new start date.
1444 also ticks the box of lots of people feeling like they're reselling the same product with a new skin (graphics, UI) for an AAA-game price tag. I also think doing some more research is easier than reinventing the Europa Universalis series, as they'd have to if they're going to have an identical start.
I think you are placing too much emphasis on how much impact a change of start date would have, nor do I think keeping EU4's 1444 start date in EU5 would render EU5 a reskin of EU4 anymore than HOI4 is a reskin of HOI3 because both come with a 1936 start date.
Besides, Johan's presence in this thread and his discussion and engagement with some points whilst keeping his statements as broad as possible is a strong hint to me that EU5 is already in development. Many people have reached this conclusion based on EU4 clearly winding down with the map freeze, tidying up exercises on game mechanics and a retreat from big expansions (Leviathan, the last one, is nearly two years old). Even the dev diary this week reinforces this point of view, as we are told about a look at the Ming and the Far East before next week's changes to the Ottomans. These are probably the last times these regions and powers will be looked at, a determination to fulfil their promise that EU4 will be in as a good a shape possible when development finally ceases. That could occur at any point now.
In other words, EU5's start date is likely already chosen. Johan has been explicit they aren't doing multiple start dates anymore. 1444 is the obvious start date as it ticks so many boxes. It could be something as you said, but I don't think changing the date for the sake of changing the date is going to happen.
And there are plenty of areas I hope EU5 I hope is different from EU4 so that it can't be accused of being a reskin, regardless of the date. I am hoping for estates and states to have a far bigger role. I am hoping for an easier to understand yet robust trade system that doesn't favour Europe. I am hoping for a development system that isn't based on spending points. I am hoping for the mana system to be removed entirely. I am hoping for a new way of colonization to work. I am hoping for a better balance between tall and wide. I am hoping for a more dynamic mission system.
In short, I am hoping that with the chance to start afresh, they can make a superior game to the one we have had. And that won't be a reskin.