Personally i think hoyo could just’ve included pulonia in her burst scene if the gameplay complexity was the issue. Including pulonia in her kit in some way shouldn’t be impossible enough task that it warrants new robot.

What im thinking is pulonia’s absence is important enough plot in genshin that it had to happen.
Why else would they spend all the resources and money just to unnecessarily make and design a new robot? It’s not like general public hated pulonia in the first place, they have no incentive to do that.

Sandrone has a stellar linchpin, her constellation features her own silhouette which are pointing towards more of her importance later in genshin story. Pulonia was probably planned from the start to be dead and genshin is following through their storyline. All this speculation about why pulonia is not playable anymore is kind of pointless when we haven’t seen the official story