TSeize is rare because power. Simple as that, you can't get that level or greater of power from anything else in that format. Period.
Wizards hasn't done anything close to TSeize because they believe it went too far. So they won't reprint it because not only does it repeat a mistake, but also because it wouldn't be new any more. They want the new cards paving the way in a standard set.
If power level determines rarity, then why has lightning bolt never been printed at rare/mythic?
Thoughtseize will never belong at mythic because there is nothing mythic about it. At the end of the day, it is still just a discard spell. Yes it is a good card. Should it be forced into rare/mythic because it is playable and highly used? No more than lightning bolt should be rare/mythic.
They're both similar. They are arguably the best 1 cmc spells of their respective types. Some decks might be able to use other cards to greater effect due to certain synergies and many decks can perform moderately well and still play the same with lesser substitutes, but at the end of the day they are the gold standard that other 1 cmc spells are compared to.
They're both cards that can comfortably be reprinted at uncommon from a gameplay perspective. For the one time Thoughtseize was printed, WotC felt that the limited environment would be healtheir if it was rare. They may have been overcautious and limited might have been fine if it would have been printed at uncommon. Bolt was orginally common.....but they really had no experience for balancing cards at the time (as evidenced by a certain blue card from the same cycle as bolt).
They're both staple cards, but bolt has had the opportunity to be printed many times while thoughtseize has only made one appearance at an overcautious rarity. I believe that WotC would agree that such a limited supply of a format's staple card is a significant problem, so I imagine that it is only a matter of (hopefully very short) time until its supply is increased with a reprinting. Not just any reprinting, but one that brings thoughtseize's supply a little closer to its staple cousins like bolt. So that means reprintings in heavily printed sets and perhaps even at lower rarities.
But this isn't just about gameplay. Despite what they say, WotC cares very much about the secondary market and they want to avoid anything that slightly resembles the problems that led to the reserved list. They won't reprint it initially at uncommon because that would crash its price overnight. When it gets reprinted, it will be at rare so that its secondary market price does not decrease by a ludicrous amount. Someday it might see print at uncommon, but not until it has been reprinted at rare several times.
If power level determines rarity, then why has lightning bolt never been printed at rare/mythic?
They're both similar. They are arguably the best 1 cmc spells of their respective types. Some decks might be able to use other cards to greater effect due to certain synergies and many decks can perform moderately well and still play the same with lesser substitutes, but at the end of the day they are the gold standard that other 1 cmc spells are compared to.
They're both cards that can comfortably be reprinted at uncommon from a gameplay perspective. For the one time Thoughtseize was printed, WotC felt that the limited environment would be healtheir if it was rare. They may have been overcautious and limited might have been fine if it would have been printed at uncommon. Bolt was orginally common.....but they really had no experience for balancing cards at the time (as evidenced by a certain blue card from the same cycle as bolt).
They're both staple cards, but bolt has had the opportunity to be printed many times while thoughtseize has only made one appearance at an overcautious rarity. I believe that WotC would agree that such a limited supply of a format's staple card is a significant problem, so I imagine that it is only a matter of (hopefully very short) time until its supply is increased with a reprinting. Not just any reprinting, but one that brings thoughtseize's supply a little closer to its staple cousins like bolt. So that means reprintings in heavily printed sets and perhaps even at lower rarities.
But this isn't just about gameplay. Despite what they say, WotC cares very much about the secondary market and they want to avoid anything that slightly resembles the problems that led to the reserved list. They won't reprint it initially at uncommon because that would crash its price overnight. When it gets reprinted, it will be at rare so that its secondary market price does not decrease by a ludicrous amount. Someday it might see print at uncommon, but not until it has been reprinted at rare several times.