This is my promise:
If I win the lottery, I'll get in touch with you in order to set in motion that plan (or a similar one)

I literally went to sleep last night wondering what it would take to create real, professional looking WWII Pacific Theater surface warfare sim using Silent Hunter 4 as the starting point.

It has pretty much all of the 3d ship models you would need as well as all of the "terrain". I know some work has been done in this regard but much more could be done, or so it seems to me.

Well, I would say that for starters you'll need the release of the source code by the developers. And since in the case of SH4 the developer is Ubisoft (if I'm not mistaken) then the chance of getting the SH4 source code would be similar to finding a needle in a haystack.

True.

Which raises the question: why, since they have such a head start using SH4/SH5 etc as the basis, doesn't Ubisoft make a surface warfare sim themselves?

Incuring the risk of sounding "too simplistic", I would say that a single word is enough to answer your question and the word is:
- Profits
Or more precisely, raw profits.

Ubisoft is different from all current Combat Flight Sim developers/publishers in which (Ubisoft) is a multi-billion dollar company. For example and according to wikipedia, Ubisoft revenue in 2016 was almost 3 Billion Euros (2.984 Billion Euros) with a net income (also in 2016) of more than a half a billion Euros (561.8 million Euros to be more precise).
I don't have such values for 1C for example but I would say that it's very, very far from Ubisoft! For example and according to the following site:
http://www.metacritic.com/feature/game-publisher-rankings-for-2016-releases

Ubisoft "only" is the 10th biggest gaming company in the world and 1C is nowhere to be seen in the list above.

Well, what I mean with this is that such huge companies (such as Ubisoft) always or almost always have to answer to shareholders which means that such companies will only develop games which are likely to give massive profit returns.
Again, this doesn't mean that Combat Flight Sims don't give profits, they do! I repeat, Combat Flight Sims generate profits, and this includes comprehensive combat flight sims such as a "1942 PAW 2"! However and unfortunately they don't generate the level of profits that the big gaming studios such as Ubisoft desire.

I understand and approve of the profit motive!

But it seems to me that Ubisoft is being a bit short sighted. They poured a lot of resources into SH-IV / SH-V and it seems to me that a way to milk that franchise for even more profit with minimal additional expenditure is to develop a surface warfare sim out of their sub sim.

Absolutely and I fully agree with you.

What you say is one of the reasons why I think and previously said that the world seems to be a messed up place nowadays. People (specially "high cadres") seem to think only about today or the "right now" and cannot seem to have the ability or the will to think "out of the box".
Resuming they want the profits today and are not willing to wait for something that would end up giving them more profits but this, only "tomorrow".

Add to this the actual trend of mainstream games in "take be hand" the palyer, with all icons, highlighs, the shoot here, go there... jumping over the screen all time.

A Ubi "CFS" will result - from "old" CFS players perspective - more disastrous than Loft failed attempt to make BoS a "mainstream" game (that XP, unlocks BS).