Just thinking aloud here. "IF" a driver can get 392K miles from a Tesla model 3, on the original battery, through proper charging methods, and avoiding excess stress, I don't see why a Rivian delivery van can't do the same. Being as the pack is nearly twice the size, and battery stress can be limited.
They are using the very same Samsung 2170 NCA (nickel cobalt aluminum) batteries that the nearly 400K mile Tesla uses.
I've heard they are charging them to 100% every night and rarely go below 60% remaining at the end of the day. I wonder if the 100% is really a software limited charge. As the thing has a range of 140 to 150 miles, with a 135KWh battery pack. Guessing yes, the charge percentage and charge rate "might" be limited to preserve the battery pack. If not, the knowledge gained in early operation might encourage better battery management.
I think it will eventually be a success. Remember the old mail trucks, (Grumman LLV is one version) they went through engines and transmissions fairly rapidly, and used a lot of fuel.
So, even if the Rivian needs a battery sooner than expected, it may not exceed the cost of engine, transmission and fuel expense vs. elec power.
But, let's be real here, the cost of the Rivian is stupidly high. Double what fleets pay for a comparable Ford.