This is true but on the flip side, saying 'people are welcome to believe what they want' is a dangerous attitude to have. Anti-vaxxers are welcome to believe what they want and this leads to death. People who believe in Traditional Chinese medicine or other alternative medicines often choose it over real treatment, and die. The more this spreads, the deadlier things become.
Flat earth is just a branch on the conspiracy tree that spreads the fllame of distrust in science and facts. The more it spreads, the more the sentiment of distrust spreads with it and we end up not only with people taking wrong medicine, but a generally misinformed society empowered by a growing collective behind them.
Does attacking allow them to grow even more via alienation? It's possible, but i'd wager simply letting everyone feel welcome to their beliefs does far worse.
Take the NRA for example. Regardless of your opinions on gun rights, the reason the NRA are eso successful is because they keep pushing their agenda day in, day out, around the clock all year, growing daily. Opponents of the NRA are only typically active when one of the thousands of mass shootings happen to go viral, then you get maybe a week or two of people trying to fight back against the NRA before turning back to their dating apps or whatever, allowing the NRA to keep growing.
The same applies here, I'd say
Wow. I love you. Seriously :-) but Id have thrown in a few 'religious people are welcome to believe in what they want' and this can most definitely lead to death.