The tradeoffs of allowing or preventing the merger are more abstract and counterfactual. We cannot know for sure what the world in which Adobe successfully acquired Figma would look like. Its natural to imagine concerns of Adobe simply killing, enshittifying or failing to improve the product - all things that still may happen under Figma's new corporate structure. Also consider the potential integrations with and improvements to Photoshop that have been missed.
That all being said I think Figma is an excellent product for the price and I have no fondness towards Adobe (though I've never really been a customer) and I'm glad that Figma exits as its own delightful product.