If I'm not mistaken, you are already granting the privilege to one group, which are donators. If you allow other groups to get benefits given to donators, then it could be a slippery slope where other groups will also demand linked account privileges to optimally complete any joint-projects that would benefit from the feature. If you are being genuine that there will be a slippery slope dilemma, then you must believe that this feature has a pragmatic value of interest to many users and also user groups. Encouraging collaboration and group participation should be a core interest for a forum board community. It's true that we have old methods that function fine. However, there is still a scale of difference between aiming for enabling users to participate on an optimal level versus having them on relying on current methods which are now considered a compromise in comparison to the privilege of having the perk to expedite the process all together. I understand that bigger avatar size and signature perks should be charge transactions, however enabling features that expedite written content should be an intrinsic value of a forum board.
Finally, I understand that the site should be profiting and don't believe it should ever be pro-bono. However, this site profits by the low-cost model via loaning donator status to users resulting in them renewing when their subscriptions expire. The initial payment is rarely the issue, but rather the idea of not effectively using the loaned status effectively during the duration it is given. Arguing that the payment model is low isn't what detracts me from placing a donation back into the site, it's that I don't know what will happen to me in a week that might cause me to be MIA on this site until maybe a month or two from now. For instance, I had premium status in the past and donated, however it expired and I understand that a feature such as linked accounts is not an available option to me. However, the idea that one goes from premium to regular is a jarring concept given that this is one of the few forum boards I've been on that don't have a permanent donator status feature.
Premium isn't cost prohibitive, it's lunch money. However, if we're arguing cost, how much effort was required to download and implement a Xenforo patch. Colors and accounts were add-ons developed by the developers or a third party. The perk isn't objectively worth the price set on it certainly if we are insinuating that worker's compensation plays a role in a subscription fee since it is a feature that doesn't need to be maintained to be effective and also wasn't coded by any of the staff. Subscription donations aren't also objectively worth the price for a site that's being driven by user contributed content. However, hosting and management of the site is worth the contribution, not an enrollment to a subscription plan, but at least a donation.