I'm surprised they haven't already invented a trendy new buzzword for this incredibly common and decades old practice so they can pretend bringing sandwiches to work is some previously unheard of phenomenon.
"BrokeBagging Latest Threat to Hospitality Sector"
Miserly lunchbaggers
Calorie-counting health-nuts
Shiftless "WFHers*"
Hardworking Restauauntiers

"You don't have savings? LMAO just buy less starbucks."
"Ok"
"NOOOO!!!! YOU'RE HURTING THE ECONOMY!!! BUY MORE STARBUCKS!!!! BUY MORE STARBUCKS!!!!"