The main brunt of the issue with Riot's decision to not let a team pick up the Guard can be boiled down to 2 things:
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They don't want Orgs to be incentivized to buy the ascension slot in the future.
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They don't want Orgs to be incentivized to sell the ascension slot in the future.
The issue here is, there is a perfectly good way to mitigate both of these worries which Riot has completely ignored.
Riot has stated that they were fully prepared to allow players to choose an org to back them if the situation where a F/A team won ascension ever arrived, meaning they have no issue with an org investing purely into the players by signing them at the end of ascension.
In the future, Riot is worried that an org like The Guard could win the spot, pay a sum of money to another organization and refuse to sign their TPA, or sell the roster (for a large premium since those players would now theoretically hold the rights to ascension).
Now, this argument is completely useless regarding the current situation, because all of the Guard players are currently free agents; this means that the Guard would not be able to sell them to any organization and the (now former) Guard Roster would be able to decide as an essentially F/A team.
But that doesn't mean that the precedent can't be stopped at the same time either.
Making an exception now won't ruin the future of challengers because Riot is able to add or remove clauses into the Team Participation Agreement that challengers orgs will be forced to sign next season if they want to compete in the Tier 2 circuit.
Next season, Riot could simply:
Add a clause to the Team Participation Agreement that they have challengers teams sign, which will release all their players' contracts should they not release their signature, and only in the case that they win Ascension. This would make the team essentially be F/A, allowing them to look for an org to pick them up.
With this, teams wouldn't be incentivized to sell their spot because they'd lose ownership over their players if they didn't release their signature, and the F/A team would decide for themselves who picks them up, removing any collusion between two interested parties wanting to buy and sell the roster.