Okay...so, MTG is like 26 years old and they basically have their own Old Testament/New Testament with the Mending........there's also the fact you could go back and sift through every scrap of MTG material and not find the answer you're looking for. It's very well possible that this is poor writing/poor knowledge of the source material or just plot convenience.
Assuming that is the case, I'll play devil's advocate and try to justify their reasoning. The best possible scenario and explanation would be that since Ugin is at or near the top of the food chain when it comes to powerful entities, his powers are left open ended within a reasonable extent. Add to that the fact that they are both elder dragons and may possibly share some unexplained relationship where Ugin is able to manipulate Bolas' ability to use his magic which would have nothing to do with the spark itself.
As you posted, Bolas knew his name(s) but couldnt say them.......Ugin says he's forfeited the right to those names AND the powers that went along with them (key point there) as well as Bolas cowering down before Ugin.......so what it sounds like to me is Ugin has put some kind of mental whammy on Bolas that has cut him off from his magic. It could be an actual spell, it could be a mental block (like what Professor X did with Jean Grey and the Phoenix Force)or it could go back to that Elder Dragon connection. Whatever the case, it doesnt seem like losing the spark is what causes the loss of magic.
As far as the how long Bolas would live afterward, I see some people saying he was already ancient way back when and we still see Ugin saying he has plenty of time left just not eternity. I don't see anywhere a set in stone amount of time he has left.......and even if Ugin said he didnt have much time left, time for a nearly immortal Elder Dragon would be much different so not much time for Bolas could be anywhere from 2000 to 5000 years on the downside. Without trying to break down and scrutinize every little detail, it's basically just saying Bolas has nothing left. No spark, no magic, no name, no freedom, no more immortality........he's lost it all. And it's not so much that he won't continue to live for a very, very , very long time by human standards, it's that he won't live forever as the Dragon God he fancies himself to be.
If he lived another million years it wouldnt be enough for him because he's the physical manifestation of evil and a large part of that is greed as evidenced by his plans to obtain unlimited power at the expense of the entire MTG multiverse.
Assuming that is the case, I'll play devil's advocate and try to justify their reasoning. The best possible scenario and explanation would be that since Ugin is at or near the top of the food chain when it comes to powerful entities, his powers are left open ended within a reasonable extent. Add to that the fact that they are both elder dragons and may possibly share some unexplained relationship where Ugin is able to manipulate Bolas' ability to use his magic which would have nothing to do with the spark itself.
As you posted, Bolas knew his name(s) but couldnt say them.......Ugin says he's forfeited the right to those names AND the powers that went along with them (key point there) as well as Bolas cowering down before Ugin.......so what it sounds like to me is Ugin has put some kind of mental whammy on Bolas that has cut him off from his magic. It could be an actual spell, it could be a mental block (like what Professor X did with Jean Grey and the Phoenix Force)or it could go back to that Elder Dragon connection. Whatever the case, it doesnt seem like losing the spark is what causes the loss of magic.
As far as the how long Bolas would live afterward, I see some people saying he was already ancient way back when and we still see Ugin saying he has plenty of time left just not eternity. I don't see anywhere a set in stone amount of time he has left.......and even if Ugin said he didnt have much time left, time for a nearly immortal Elder Dragon would be much different so not much time for Bolas could be anywhere from 2000 to 5000 years on the downside. Without trying to break down and scrutinize every little detail, it's basically just saying Bolas has nothing left. No spark, no magic, no name, no freedom, no more immortality........he's lost it all. And it's not so much that he won't continue to live for a very, very , very long time by human standards, it's that he won't live forever as the Dragon God he fancies himself to be.
If he lived another million years it wouldnt be enough for him because he's the physical manifestation of evil and a large part of that is greed as evidenced by his plans to obtain unlimited power at the expense of the entire MTG multiverse.