Why I'm not worried that mana potions are getting cut

The reason why is that there will most likely be a starting item to replace it. Take a moment to read this:

Starting Items In-hand with our goal to offer a diverse preseason, we decided to take a gander at starting items. When there are standard starting item builds for every game of League, the end result is these builds tend to benefit (very slightly) certain champions over others. We’re taking a different approach this preseason by offering more clear opening ‘paths’ so that players can refine their playstyles with real choices, rather than just opting for a generally powerful start. You’ll see these changes reflected in two sets of items: starting power items, and sustain. For items you’re bringing to lane, we’re using the Doran’s set (blade, ring, and shield) as a baseline “generalist” group that provide all the broad stats you want. From there, we expanded your options to provide a few more ‘specialized’ approaches, like Cull - a light skirmishing AD starting item that provides less defensive stats than Doran’s Blade, but gives bonus gold for killing a set number of minions.

I bolded what I thought was most important. As you saw, there is a sustain category for the new starting items. Doran's Ring already offers mana. So what that means to me is that there will be an item like Doran's Ring that either offers less AP and/or HP, but offers more mana regen and/or health regen. Basically, it's gonna be the item for AP mages that want some basic regen before they have to back for an early tear.

So basically, stop worrying. We still have time before we have to start panicking about these changes.