It just doesn't feel complete without them.
Everything in the game pre-DLC had a Spirit, and some DLC content is getting Spirits, but other DLC content isn't. And they DO periodically add additional Spirits(Mario Party, Labo, Link's Awakening, etc.) So if they're going to add music from franchises that are otherwise totally unrepresented in Smash, I don't know why they couldn't throw in some Spirits too.
Spirit battles don't seem like much effort is required given that they're made up entirely of pre-existing assets. Characters already in the game, with modifiers that are already in the game, on a stage that's already in the game, with music that's already in the game, using a jpeg that was probably made a decade ago.
Nintendo likely realized one thing though: Spirits are completely borken. And it's not just Spirit combinations (like One Punch Mac) that's broken. Spirit-related currency (SP) can also be easily broken and turned into an unstoppable snowball of gaining infinite currency.
There is one main reason for this: you can sell Spirits
twice. Once by Dismissing (which gives you the Spirit's core and some SP depending on the Spirit's Stars.) And next, by selling the Spirit's core by going into the Inventory - > Items menu and selling the core there. You can get a very early 3 Star Spirit or a max level 2 Star by just beating 10-15 spirits in WoL and selling them (plus unlocking two fighters). Either of those allow you to beat more Spirits, means more sold, means more SP to upgrade or buy more powerful Spirits, rinse, repeat. Soon you'll have at least one broken Spirit combo and WoL's difficulty snaps.
No wonder Ninty just gave up on balancing Spirits and A) made it so DLC spirits are given to you without that minigame, and B) made it so you could get a very powerful Spirit by just getting a 3-7 win streak on Spectate. Oh, and you can also buy SP from Spectate.
Also, Spirits are not the only way to promote something - Sans' Mii Costume did an excellent job of that by just being announced. So yeah, Spirits have become completely optional.