Sounds like you don’t really know what you are talking about …. as a many retail systems - whilst not ‘green screen mainframes’ - do have many Central/Enterprise functions such as order management , price and product data replication, cash management, Enterprise Returns, Warehouse Management, Meechandising, Logistics
… based on Windows, SQL Server and in recent times SaaS and micro services/containerised architectures.
How do I know… as I work for one.
The secret is building in resilience and robustness to outages like AWS or comms failures so stores can continue to trade offline if required. Whereas if Facebook or Office 365 has an outage everyone is fucked until it’s fixed.