You talk about replacing his bat, but the Nats would only not make a strong attempt to resign Murph if they feel he's not likely to maintain this level of play into his mid 30s. So if Murph isn't going to be the same Murph, then you in theory have to "replace" his bat as he is today regardless if you have Murph or not. Just having the name Daniel Murphy still in the lineup in 2019 and beyond doesn't mean you avoid replacing the current All Star we have.
Rizzo was smart to let Zimmermann go and not give him big long-term dollars. Desmond had a good first half in 2016, but we also dodged a bullet not signing him long-term. I'm sure they will see what Murph looks like physically next year before deciding on what if anything to offer him in the form of an extension. We could buy Face out after 2019 for 2 million, so that does mean we could hope Murph can play 2B in 2019 and then move to 1B for a few years after that. If we signed him to a 3 year extension, he'd play 2B in 2019 and then 1B in 2020 and 21.