Innovation will not stop, since this isn't the phone system. However, the basic protocol has been stable for quite a while and is sufficient to implement all basic phone services. Enhancements tend to be for specialized services, such as ISUP interworking, QoS negotiation, liveness detection, caller preferences or presence/instant messaging. All of these are backward-compatible with the basic protocol, with extensions negotiated if both sides support them. Basic calls will succeed without the extensions.
Note also that just because something is proposed doesn't mean it will ever become a standard. (Only about 10% of Internet drafts ever get published as RFCs.)
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SIP FAQ : 
What is not described in the SIP spec?