First of all, remember that these are PBE concepts. They will test them and many changes will likely get reverted.
It is easy to pick apart each item and see it as a nerf. But RIOT is looking at item builds holistically; What they provide, what the build paths look like, and the intended uses for each item. If you look at the new AP item ecosystem, I think its really a buff.
Mages have been in a poor place itemization wise for quite a while now, and we've learned to cope with more powerful champions by relying too heavily on specific niche items (aka zhonyas). Some of these items were never intended to be your go-to item for ever single game, but rather a more situational items. Rather than doubling down on niche items RIOT is trying to solve the root problems for mages. Lack of overall Mana (having to build MP5 in order to cope), Poor CDR build paths (not having your spells up frequently enough to deal with these champs), and lack of mobility options to be able to avoid damage and or counterplay. Rather than being a big ball of AP with limited ability to counterplay (other than blow their shit up), they are taking off some of the AP to compensate you with more skirmishing options.
Now, Granted, they've already identified some of these changes are very experimental and will likely be changed. Don't get too focused on the actual numbers right now, focus on the fundamental shift they are pushing for. Slightly less AP and MP5 for more Mana, better build paths, more cdr, potentially more mobility.