Lets analyze the verse again, [21:35]We granted not everlasting life to any human being before thee. If then thou shouldst die, shall they live *here *for ever? It just amazes me that just to keep Jesus AS alive, to what extent one can really go. Anyway, Glad you did not say that it was incorrect interpretation. Now, taking your explanation on this verse, Jesus AS was born around 600 years before Muhammad PBUH, you and I both agree Jesus AS is/was a human being. I ask you, what was special in Jesus AS which lead Him to have longer life than holy prophet PBUH?.. A simple logic tells me that He AS was born 600 years before Muhammad PBUH and i see no reason to believe why Jesus AS was raised up and Muhammad PBUH was given death. Theres an age limit of human's life cycle. One cant pass that limit. Prophets have lived for as long as 300 years of age, but eventually died, in this very earth. You do know very well, the verse you're trying to give explanation of can by no mean support the belief of Jesus AS being raised up alive.
If something is obvious, it doesnt require explanation. You cannot take a verse and add your 2 cents in it, to support your belief. When Muhammad PBUH was given death, Jesus AS is no different. He AS did nothing extraordinary if compared with Muhammad PBUH.
Jesus AS is living bodily in the heavens for more than 2000 years, this to me doesnt seem the longevity of life but rather considering someone being immortal, given that there is no specific time of his arrival according to you.
Also, have a look at this verse and the verse you're explaining and give some time to think about it..
3:144 ( or 145 for some )And Muhammad is only a Messenger. Verily, all Messengers have passed away before him. If then he die or be slain, will you turn back on your heels? And he who turns back on his heels shall not harm Allah at all. And Allah will certainly reward the grateful.
one hand Allah says he didnt grand anyone everlasting life, and on other hand allah says all prophets have passed away ( even if i take your interpreation of many and not all , it then requires one to see the verse in which the mentioning of Jesus AS used to eat food , it will leave you with no doubt of his death., though the correct interpretation is that all prophets have passed away before him )..
Do you really not think this explanation of yours gives birth to a lot of questions? Some of which is, if Jesus AS was alive even during Muhammad PBUH's prophethood, what was the need of Allah sending down a brand new prophet and not sending Isa AS back to earth?.. is it possible that Jesus AS have learnt Quran even before it was revealed to Muhamad PBUH, since Jesus AS is/was the closest prophet to Allah? Does questions like this seriously not come in your mind when you're trying to explain a verse ?
At least accept that your argument of dying before Prophet Muhammad(SAW) has nothing to do with the ayah you quoted above about immortality.
Ashab e Kahf slept slept for hundreds of years and stayed alive, it doesn't mean that they were better than the Prophet Isa(AS) and Muhammad(SAW). Similarly, Isa(AS) being taken up to heaven alive doesn't mean that he was better than those who weren't taken up alive.
Now as to your question why Allah took him alive, look at Isa(AS)'s situation, he was in danger and Allah didn't want him to be murdered so He the Almighty saved him by raising him alive.
Now can you stay on topic by talking in the light of Quran and hadith instead of in the light of your own logic.