Well, now Odyssey offers something of value to you - Ships.
You could buy Odyssey to have access to Python Mk2 and the rest of the ships
Or you can keep limiting yourself to the base game and buy ships for Arx - it may be cheaper for just one ship, but they plan to release 4 ships, possible more, which means it's cheaper to just get Odyssey.
Now, back then before Odyssey, when the game was split in Base game and Horizons, there were at least 8 ships that you could buy only in Horizons.
No Horizons, no ships - there was no Arx option back then. And the ships were quite significant: Dolphin, Beluga, T-10, The 3 Alliance Ships and the 2 Kraits.