It depends on what you need/want.
You're right that you could use a ESP8266 or even better a ESP32 as a serial to wifi bridge, however if you add a raw capable M8T or a modded M8N GPS and RTKLIB on the pi you should be able to get RTK working as well. One of the main reasons we use the pi is to run mavproxy, which gives us the ability to reroute telemetry over RFD900x, wifi and 3G/4G mobile (USB dongle) and even back to a Taranis screen, however we want. Also imaging etc can be recieved via the 4G modem and multiple GCSs can be used, like tablet or PC. Another thing you can then do is to run a I2C oled to monitor link status directly and integrate a few buttons as a failsafe, for example one for RTL, land, and play/pause etc. Or run Ardutracker for a directional antenna. It opens up a fair few options that need to be only installed on linux on a pi, but would need to be ported to ESP32 etc.