Wholesale rate is for businesses where they pay for the actual usage, and consumer-oriented providers, which you mentioned, oversell network capacity.

There's overselling and then there's assuming that the average user is only going to use 3% of their allowance! That seems pretty optimistic. Having said that presumably the wholesale rate varies quite a bit between countries, and I assume some contracts are offered as loss leaders.

One of my relatives in Italy is using Kena. It's solid. Even though they advertise 30 Mbps, we clocked it at 68 once. Generally it's more than enough to watch Youtube, which is where most bandwidth is spent, unless you hotspot to a laptop.

The point about the overselling / oversubscribing for consumer rates actually goes counter to your point since every customer (consumer or business) all have the same priority in European networks.

€4.50/GB is the maximum what EU operators are allowed to charge each other for roaming data transit right now, and what an external operator like Twilio would probably have to pay them. Of course in each country you will find operators who offer better deals on their own networks - but Twilio can't reasonably build out their own worldwide mobile network.

Despite this, I can see this service being popular among companies who are willing to pay a bit more per GB to avoid negotiating with operators and getting SIM cards all over the world.

Depends on the country, no? In Spain you get like 1 to 3 GB (depending on the operator) for 7€. And that's with very cheap operators with poor support.