a16z's Alex Rampell says the best software incumbents have hostages, not customers: "Normally, selling into the brownfield is hard just because it's brown. What does brownfield mean? It means that it's been trampled by an incumbent." "Try selling ERP and you have a product that's much, much better than SAP. There's a saying Joe's heard me use a million times: 'The best companies have hostages, not customers.' SAP has hostages. That's a brownfield." "Sometimes you can break in. Cloud versus…