Right.
Now extend that to include a meta-BMS.
If you've got a dozen 100a all under a meta-BMS you can reasonably contemplate a 500a max pack rating.
The mbms keeps a close eye. If one pops, alarm. If two pop shut things down with ATC/ATD. Maybe detune your max slightly. You can set the mbms threshold wherever you want.
Edit to add: if that were to occur it might call for pack balancing rather than battery replacement.
Further edit: a smart mbms could handle this.
Wow, I'm being allowed edits for a long time.
Let's extend the commodity approach. What could you do with 100 ah 50a BMS?
I think it all comes down to the reasons why a battery's BMS disconnects, how you diagnose it, and what's involved to bring it back on line or replace it.
One battery disconnecting in a bank doesn't mean the battery is bad. It might, but it might not. It could just have a cell that's out of balance and hasn't had enough time on the balancer to correct it. The people I mentioned who were cutting open drop-ins and adding their own BMS was largely motivated by just this. Apparently some of the drop-ins can't really keep up with balancing.
The disconnect could also be due to over current in one battery. The current in parallel batteries will never be exactly the same, and might vary quite a bit. This means that one battery will always be the first to trip. It doesn't mean the battery is bad. Also note that in this situation you are likely to get a cascading shutdown since when one battery disconnects from an overload, it leaves the others with a larger load and will likely cause another to trip, etc.
There are also temp out of range, and low voltage shutdowns to consider.
Then you need to know that the battery disconnected. With most drop-ins you can't even tell that it disconnected without removing it from the bank and testing it. Let's say you have per-battery disconnect switches so you can periodically test the batteries one at a time. And if it has disconnected, you have no way of knowing why. You could measure voltage and perhaps get insight from that, but it's not a certainty.
You mentioned some sort of bluetooth connected app in some batteries as a means of visibility into each battery. That would help a lot and let you do a periodic check to find a battery that has disconnected, and presumably find out why it disconnected.
Now let's look at recovery and repair. If the fault is believed to be transient, then you are ready to put it back in service. Otherwise you need to replace the battery. After that, you need to reintroduce the battery back into the bank, but they will be at different SOCs. So you need to synchronize them, probably by bring the bank to full charge, and separately bringing the replacement battery to full charge. Then you can through the switch and merge the battery back into the bank.
I'm not saying this an unsolvable problems. Not at all. Just pointing out all that's involved, just in case less experienced people see it as a simple solution.
I have to say that it all seems a lot more complicated than just building with a fully functional BMS in the first place. Are drop-ins really that much less expensive? So much effort goes into working around the limitations of drop-ins, rather than more directly accepting and working with LFP battery characteristics. Drop-ins seem easier, but always seem to turn into one complex work-around after another.