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↩ characterslimits You only want to switch because of nostalgia. I get it, you want to relive the old days and change sucks.
The desire to change isn't driven primarily on nostalgia. @"videogamesm12"#4 wrote a highly detailed explanation on why proboards is more desirable than the current setup on flarum which from what I can observe, the major consensus here is that it is absolute garbage. As @"Telesphoreo"#36 also said, despite the community having severe objections to moving to a different platform, the reason was “whether you like it or not it’s happening” and a cryptic announcement of the fact was made by the previous owner, Seth. This announcement would result in the eventual adoption of flarum following Seth's departure.
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↩ characterslimits Forced advertisements
It's a free service so they have to get revenue through something to keep it free. If you don't like the ads, I highly suggest using an adblocker.
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↩ characterslimits Forced “shop” feature (bloatware, additionally it makes our community less child-friendly, kids will start accidently buying virtual items on the forum with real-life money as they don’t know the concept of money)
Tell that to every server that has an in-game shop where kids could 'accidentally' buy virtual items and make it very easy to purchase said items. The items are not mandatory and to my knowledge, most OPs didn't buy them anyways. Also, the location where you can purchase the items is a separate tab on your profile, and you have to click a button to go to the actual page to buy said items.

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↩ characterslimits Increased unappealable bans, as we will have to start enforcing ProBoards TOS (violaing the TOS of any platform is a Catagory 0 ban)
The discord already has a ToS and there's been like one documented case of someone being ToS banned that I personally know of, so it isn't like regular indefinite bans which are handed out due to players crashing/attempting to crash the server on a weekly basis. Truly unappealable bans are generally issued for severe rule violations, such as breaking actual law. Also, if someone was determined enough, they can easily just lie (Not endorsing this) about their age. This isn't the TSA where you have to show ID every time you log into the forums so it would be very hard to prove someone's age unless they were being honest or make it very obvious they are underage through their behaviour.
I am going to assume you clearly have never used proboards to the fullest extent to understand why people like it so much - it is reliable, easy to use, and actually has features you'd expect from a forums software suite that does not require a metric ton of 3rd-party plugins to obtain said functionality.