Uh, no. Data loss for your end-users as a default behavior is unforgivable. The impetus is on people who think it's OK to irretrievably lose access to their freaking data when their phone battery explodes to set that up. For myself, and for Joe P. Random, losing my data without my having explicitly (and again) decided to take that risk is what directly precedes a decision to no longer use any Google products, full stop.

People already assume their voice conversations are (usually) ephemeral. As long as it is explained properly, an ephemeral text chat can not only be acceptable, it may also be a feature.

More importantly, why are you pretending that end-to-end encryption implies an irretrievable loss of data? If desired, backups can be made locally or to another local device such some type of LAN-only network storage or a USB storage device.

Exactly. Snapchat = ephemeral video sharing/chat. Why is it so big?

Exactly, and this is precisely why it's E2E by default with iMessage. One company makes money off of your information, the other makes money off your hardware.

That is how Allo works.