Some of my early phones had spare batteries. They most certainly did NOT have weeks of standby time or 20 hours of talk time. We are talking late 90's.

Later, as phones and batteries got better, the spare batteries became unnecessary. They still degraded fast enough that there was a market for replacement batteries and they could indeed easily be replaced. We are talking things like the Nokia 3310.

Even later, the need for user replaceable batteries pretty much disappeared.

These days, it is entirely gone.