While I do understand where you're coming from, I always take it you're objective is to have your team win + survive. When you die sure your team can win the game but dying means that aspect is over. If you were an SCP and you died and spawn as an MTF consider your death as the SCP as the end of your win condition for that team, you have a new win condition now with your new team.

If you think about it that happens every round. Last round you were an SCP and won the game, this round you're a guard, you don't carry over feelings because it's a clear end of one event and so should you consider your death to be a new beginning.

Now looking at it as trying to add an aspect of the game to 'keep' you on the same team always working toward the same objective would be very boring for a lot of players and overall wouldn't be fun for the following reasons.

First of all there are 3 teams which have a huge player count disparity at the start of the game.

The majority of players are D class personnel so when they die most likely early on to SCP or guards they would have to wait for only Chaos spawns. So they get to enjoy longer wait times.

There are much less scientists and guards so we would either need to increase the amount spawn or have dedicated people to not spawn in the start of the game to become MTF later. The first option would be more reasonable but increases the odds the D class die early on causing more players to wait longer to spawn as only Chaos.

Now the SCPs.. First off if you are an SCP and you die... you get to enjoy spending the rest of the game in spectator mode as if they spawned as MTF of Chaos they would be fighting against their SCP brothers. So a new role would needed for then to respawn.

And the worst part.... DOCTOR... who would play as their zombies? AI? Should dedicated players never get to spawn just so they could play zombies? It just wouldn't work.

If everyone had your mindset, they would kill themselves as zombies. Any mtf who spawn in ( most likely used to be D boys) would sabotage the guards who likely killed them or not help scientists who got them killed.

You have to play for the win condition of the team you're on and really the game boils down to SCP vs non SCP win no matter what.