Might be a bit extreme saying time to ditch old routers, Cisco (in standard IOS) still don't have this implemented, and last time I saw on the road map it was scheduled for Q2 next year I believe (those running XR etc.. have the support)
Ultimately, it's not going to break your access to networks that run with a 4byte ASN, as a special ASN has been reserved to assist in the transition (so, when you see AS23456, that means someone is actually using a 4byte and you aren't supporting 4 byte ASNs).
The downsides to not actually seeing the 4byte ASN - lack of really easy visibility of originating ASN, AS path filtering challenges etc...
Oh, and if someone with a 4byte ASN decides to connect to you directly, you're going to struggle if you aren't able to support it
Either way, many people been pushing back on their vendors (vendor C has been relatively slow in responding

) although I, and a fair few others have been pushing this on them for at least the last year actively, and probably longer actually.