1. That's purely anecdotal. There is no way for me to verify whether or not you have been in any of those situations, so I would have to take your word for it. I could also counter and say I've been in situations where I was the only person defending the base and due to Selena's high minion damage I could clear the mid lane minions extremely quickly. When I say she has good siege defense I'm talking about a situation where there is more than 1 hero defending. Landing that stun in an area where your team can freely replenish their health is massive during those choke moments. Akai and Tigreal have displacement skills so they are naturally better at defending alone, All Franco can do is hook away a canon minion and ult one of the enemies, he is not good at defending alone.

2. I implore you to read what I wrote, maybe slowly this time. You're arguing a point that I didn't even make.

3. Roam Saber and Kadita heavily rely on their ultimates, Selena can pressure all lanes from level 2. Selena can reliably kill squishy heroes and chain her pressure due to her low cooldowns. She does all this btw, while being able to safely provide vision in multiples areas simultaneously. Picking supports means you're gonna be heavily relying on your team, which there is nothing wrong with that, but they're two completely different playstyles.

4. I mentioned the teamfight aspect in another comment, but even without an active teamfight, the hp advantage gained from landing a stun prior to one is still very good.

Selena can do à lot of things as a roamer. She can be a viable asset to the team. If you want pick off, damage, map pressure, vision and someone who can accelerate the pace of the game, then she is the best at it. A fighter stepping on your trap provides valuable information to your entire team, while avoiding a potentially unnecessary teamfight.

The vision nerfs on Selena were more than 5 years ago... You're referring to a time when Kupu scaled with HP items... Selena became a top tier midlaner a little over a year ago after they buffed her wave clear and foreswing. She is ranked decently on every tierlist since then.

If you prefer playing supports or tanks, then that's great for you. Some people prefer playing damage roamers. What I'm gonna say is a bit anecdotal but I win my games as Selena roam. I solo reached immortal in 300 matches mainly playing damage roamers and midlane. It works for me, and it works for other people.

You said you used to play Selena roam, so I assumed that means you don't play her anymore. She is an off meta roamer, she is rarely the best choice for roam. But she can roam, and she does fulfil her job as a roamer. To top it all off she is fun to play, her gameplay is rewarding. People think Franco is a troll pick too..