Assuming you were doing dynamic allocation vs. hard allocation on the SMS you should find equivalent scaling for the form factor in the 7200/7300 series.

Typically, the smaller SMS can replaced with one or more Cisco 7x0x with NPEGx with equivalent PA interfaces (where x determines this need).

If you outline how much PPPoE/A/oE/oA you are doing and the configuration of the SMS chassis you have now that would be helpful.

In practice, other BRAS options in the market exist but it sounds like you are concerned with the EoS of the SMS series you listed. This is a Cisco forum so...