This is a short essay explaining why Fip needs a 4th attunement slot. It will show that Fip is locked into a limit on power that will only be able to be increased with either another slot or the addition of a very rare or higher magic item.
After the completion of the Witchlight arc and being given a birthday present. Fip filled his three slots. Because of this, there are few if any magic items that he would want unless they are direct upgrades to what he currently has. Looking at what he owns, the only real upgrade right now would be a staff of power to replace his wand of fireball. This takes away from the joy of the character because the likelihood of better loot dropping for Fip is unlikely at this time.
I feel this will not change the overall power of the game to much due to our higher level and the fact that everyone in the party has the option of getting the extra slot if they want. There is no problem with giving everyone extra power because that just lets the DM be able to use stronger monsters or encounters to balance out the power of the party.
My final point is that the creators of the coding of dndbeyond saw a need for the ability to let DMs and Players have more than the stander three slots. If they went through that extra work, she would consider using it.