"Starboard Value Hedge Fund" doesn't really qualify as an 'activist'. They are an investment advisory service and are a major private equity investor.
"Activist" implies there is some nefarious or PC reasoning behind this versus a suggestion for financial improvement from a financial expert.
Yeah, I didn't particularly like the headline, either—if it were just an activist, and there are plenty of those around these days, this would just be "Ridiculous News of the Day" fodder. This is actually an investor coming in, buying a chunk of the company and deciding they want to mess with its business model (ahem, Eddie Lampert).
A reminder: Dollar Tree also owns the Family Dollar chain, which operates under this guy's proposed model already.
Dollar Tree has managed to stay upright because a lot of the things it sells haven't kept pace with inflation. The trinkets and small housewares it sells haven't changed in price all that much, compared to the stuff that's driving inflation (medical expenses and tuition for education). The food items that are inflating, they can shrink the packaging, as they have for eons. They still have a couple decades to go before it'll be impossible to maintain their business model.
