I wondered how all that expensive hiring would impact employee's feelings related to working at Meta as to how much money the new AI staff is getting paid relative to their own pay.

Imagine this, you worked for Meta for X long time then you look over at your colleague and she/he is earning multiple times your highest income and they just stared. Mmm, talk about a complicated working environment.

these employees should understand that if they would have delivered, the new hires would not have taken their place. At some level your pay is not tied to seniority, but impact on the organization. If the new hires do not deliver, they will see the door faster than anyone else in the company.