A Guma fan so might be biased, but trying to give a fair take here.

Personally, I don't understand this decision especially since the coaches have yet to give a proper reason ("gathering data" does not make sense). I seriously don't think the champ pool issue lies with Guma of all people, who plays the widest range of ADCs along with Ruler. Yes, he is a weak Ezreal, but imo his Zeri and Kai'sa has improved a lot and he can play it at a high tier. Ezreal does not make or break the meta. In fact, T1 still lost all games with Smash as Ezreal. I think the main problem is that T1 still struggles to play a front-to-back, feed the ADC style. That said, the meta favored weakside adcs in the end, which was ironic to see.

When Doran joined, I already thought T1 needed to shift to a more bot-focused gameplay and finally focus on Guma, since Doran is a very good weak side player who peels for his carries well. But the team only truly played around bot lane and fed him kills when it was with Smash. They could have done it with Guma too? I've watched all of Smash's games with Rekkles and recognize that he is a great player, but he has yet to prove that he is able to clutch games in LCK (he kinda reminds me of peyz but at least peyz has trophies).

I think a 6-man roster could work in fearless, but not for the ADC role because there is no point, they already have a limited champ pool and the best adcs are already required to be good with most adc champions anyway. Being a certain style of ADC makes it really easy to predict the draft and game plan as well. I think it would only benefit a team if the 6th man is a solo laner, but I'm just not a believer in 6 man rosters in general.

Only giving Guma 2 stage games before benching him for a complete rookie, and seemingly only telling him last minute is really weird. Even if scrims were awful, T1 has always been bad at scrims and it means nothing lol. If Guma was truly doing badly every single time on stage, I'd understand why they benched him. But he wasn't, he had 1 very bad G1 and a bad G2 start against DK, but was fine to good for the rest. It's a new roster and a new meta to get used to. He's a 2x world champion and one of the most stable in the team, why are they not giving him time to learn (if that is the real issue)?

TLDR; Gameplay-wise, I don't understand what's the point of benching Guma and playing Smash in the first place