Time Capsule is not a modem and so has absolutely nothing to do with the Spec for DOCSIS 3.0
Your ISP is probably out of IPv4 addresses which are exhausted in the US.. upgrades will move much faster now to IPv6 and so new equipment is going to be required for many people.
http://www.itwire.com/it-industry-news/telecoms-and-nbn/68521-north-america-runs -out-of-ipv4-addresses
That is just the modem and is not a hugely expensive item.. the TC is fine for future use with the new modem.
My concern is, Security... Do I need DOCSIS 3.0 for security purposes or is it just a speed thing?
Normally if the ISP tells you an upgrade is required .. since new standard should be backwards compatible .. something might be different.. it has nothing to do with security.. and speed is only a minor factor.. they might be moving to new standard and it might be a real requirement. Talk to your ISP.