This reminds me of the switch from circuits to packet-switched networks. Basically what happened is that telco operators only wanted to sell circuits which, like a circuit board, were essentially a closed loop. This is why t1’s were always dedicated 1.5mbps symmetrical links that were basically unshared bandwidth. You got 1.5mbps because that’s what att provisioned (until they started over provisioning the headend/noc and sold ‘shared T1’s’, but that’s a different topic).

What happened as I understand the lore is that one year a NANOG you had a bunch of young network operators basically get together and say “well, we’re done with this circuit stuff, we’re moving to packets where it’s a network of routers that make decisions about how each chunk of data is moved through the network based on headers.” The telco operators laughed at them because packet routing is much harder to guarantee deliverability of data than a circuit and they didn’t understand why people would want cheaper unreliable data routing for their internet service.

Of course, over time, packet routing has won for everyone except the largest orgs who have dedicated circuits that aren’t shared bandwidth. For anyone that’s used a direct 10GB fiber line that isn’t shared vs a cable modem that is overprovisioned to hell, the dedicated circuit is a whole different class of internet access (and 100-1000x the cost).

SIP vs dedicated trunks is kinda the same thing.