After joining Instacart more than three years ago, Chief Executive Officer Fidji Simo had to earn buy-in from staff about her vision to reimagine the grocery-delivery company as a provider of e-commerce technology for supermarkets. Now, she also needs to convince Wall Street skeptics. The pandemic-fueled valuation boom that Instacart benefited from has since dwindled as Americans emerged from lockdowns and returned to in-store shopping.