To expand a bit on the information Laurent provided, the default decrement wouldn't be sufficient, if used, because the priorities are configured and they're more than 10 apart in your post.
Tracking the WAN link, and moving the active gateway, might not be required for correct functioning, although it might be more traffic flow efficient in avoiding the need to transit the original gateway router.
PS:
If these routers in question are recent ISRs, you might have other virtual gateways options, such as MHSRP or GLBP to better take advantage of outbound WAN bandwidths.
Also, original interface tracking watched interface up/down, but some later interfaces stay up but the path is bad. There are newer methods to also track whether the path appears good vs. just the link.
