A lot of the answers to your questions are in the article link I provided. Perhaps you missed the link, here is again:
The 7mm Practical. A Practical Magnum.
As for Remington and Winchester getting it wrong. Things are never quite that black and white. When the 7mm Rem mag was in design phase, Remington engineers pretty much had two main contendors, Les Bowman's .338 Win mag necked down to 7mm or Warren Page's favorite, the 7mm Super Mashburn Magnum which is basically the 7mm-300 Win Mag, although in those days it would have been a blown out H&H. You have to remember that H1000 wasn't around back then so the 7mm-338 Win mag was probably a lot more efficient and effective, hence the adoption of the 7mm-338 as the 7mm Rem mag.
In modern times the market has been a bit tougher. It would not be easy to convince buyers that the 7mm-300 Win mag is the bees knees and a must have. The WSM's and RUM's are vastly different from the traditional belted mags so they get plenty of attention, the premise is based on sales way before ballistic performance.
I have just, 20 minutes ago, finished my third 7mm-300 for a client. This one is shooting groups of between .420" and .465", driving the 168gr VLD at 3212fps. I will leave it at that as the barrel has yet to be fully run in.