Beginning of S1 is sketchy as fuck because Bartlett (Martin Sheen) was never meant to be a recurring character, the show was initially about West Wing's staff, and John Spencer's character (Leo) was initally meant to be THE lead.
Well, the first part is right anyway, Lowe was meant to be the character things revolved around. He was the star name of the then regular cast (as you said Sheen was meant to be a bit part) and the majority of the meaningful storylines would have him involved.
As everyone knows the test audience apparently pretty much stood up and dropped their pants for a wank when Sheen appeared and did his "Fuck you religious fools!" speech so bam, instant show retool. Sorkin "left" on the point of being fired as he was nose deep into his cocaine habit at that point and had completely lost any coherence. Lowe left in season 4 as from the point Sheen was made the main man they also started giving Josh's character more of the stories Lowe was expecting. The rest of the cast were also pissed that allegedly Lowe was making way more than them but it turned out years later they had already passed him after their first salary hike never mind the second.
It was amusing to see that it wasn't until Sorkin left that the Republicans got anyone that wasn't a cartoon villain. The show finishing when it did was always the best time to go out (despite the Fringe alternate universe making it to at least season eleven) with Sheen leaving office.
Comparing the end of the show to the start is amusing in a different way, by the end point it appears the West Wing is being run by CJ and Will with everyone else having left and none of their characters ever being replaced during Season 6 & 7. Their job role simply disappears when they do!
As for the Newsroom, I'll watch it but mostly because I'll be rooting for the Sasquatch news item to finally get shown!
I loved the armchair critics shouting about how fucking stupid the show was in depicting a newsroom early in season one, how wrong they were, it would never happen blah blah blah until Dan Rather was interviewed and said it was almost exactly right