Wonder if I can get 20 years out of a banana pi? 😆
Well If the hardware doesn’t fail - something that also happens to the WRT54G - you might. After all the hardware is way more open, there aren’t any MIPS shenanigans and other restrictions that the manufacturer may have added.
Another great hardware (not high end, mid range, older, but still solid) for you is the Netgear R7000 (dd-wrt only) or the R7800 (supports OpenWRT). I personally would avoid DD-WRT and stick with OpenWRT as supported devices are more open (no proprietary drivers thus more future support). Check OpenWrt’s supported hardware tables.
If you’re looking for new hardware to use as an AP (no USB) the Netgear WAX206 is also supported. openwrt.org/toh/netgear/wax206