I can't deny that he does bring the 'X-Factor' as you put it, but I also don't think there's a lack of aggressive middle order players in County cricket who could attempt to fill that void. Don't get me wrong, perhaps Pietersen is somewhat unique in English cricket in terms of the way he plays, but I don't think it's something that is crucial to the success of a Test match team. Plus, I do genuinely think there are guys in County cricket who can play aggressively and somewhat fill that role, who just need the chance.
As you said KP is unique to English cricket now and historically. Only Walter Hammond and Ted Dexter of all the great English batsmen has had special abilities as a batsman.
In county cricket the back-up batsmen really pushing for a place now are Compton and Hildreth, while Morgan and Bopara are outside options Compton or Hidreth on form would be solid replacement who could certainly score runs @ test level - but none have the ability to score hundreds like in the style and substance like KP did in this test and vs Sri Lanka last winter.
An England middle-order without KP, would look like the England middle-order of 1951-1958 when the team was # 1 without Denis Compton. No X-factor.
You're making it sound like we'd suddenly turn to rubbish if KP was no longer in the team. Yes, he's a decent Test match player, but he's not even the best batsman in the team, so I don't think it'd quite be the catastrophe you're making it out to be, even if it did make us less exciting to watch.
KP is the best batsman in England's team. Let not kid ourselves. This a man who style has being compared to the master blaster, King Viv Richards at his peak. He is the only batsman now who would come close to be considered for a place in an All-time England test XI. Cook, Trott and Bell would not by any stretch - regardless of the fact that are deemed to be more reliable and may have scored more runs in recent years than him.
As i stated above, i don't believe England would be collapse if we loose him. But the team would loose its flair/x-factor and be a boring team who wins.
It seems to me though, that KP wants to pick and choose what games he plays. Now personally, I have no problem with that if it's a certain form of the game. I.e. if he wanted to quit Tests and just play T20s, that would be fine with me personally. But if he wants to play in the IPL every year and play Tests, then a conflict exists. If he's asking to miss certain Tests against the smaller sides that we play at the start of the summer, yet be allowed back in for the Ashes etc, then that to me is wrong and shouldn't be allowed to happen. He needs to commit to a form of the game and be available for all those games. If he can't, then as I said, I wish him well, say thank you for being a good player for us and he can go enjoy his life, while we bring in a talented player who would give anything to play for England.
I don't see the big deal if KP wants to miss the NZ tests next year to play a full IPL before the Ashes. NZ probably won't be sending some its best players too - the series is likely to be watered down anyway - so let him go.
Wanting to miss that series doesn't show a lack of commitment to England. KP like Chris Gayle in the modern age of T20 leagues and massive bucks is one of the most sought after players and at his age, he is in his rights to recognize his value.
It's not KP fault that the ICC is broken and has a ridiculously packed international schedule, that is compounded by the fact that BCCI has manipulated them and created the IPL/Champions League to further cram the calendar.
KP and other players in his position can't be blamed for wanting to take the easy way out.
So essentially this thing is bigger than KP vs ECB, as the Chris Gayle situation was bigger than him vs WICB and Bond vs NZL vs ICL.
This is world cricket problem, that the best players in each nations most sought after T20 players will always have this problem, unless the ICC is becomes a proper governing body and stops getting bullied by the BCCI.