The US has many AB Flight I and LA class SSN waiting to be decommissioned, the Ohio class and Ticonderoga class are also getting aged, and their naval fleet's number will start reducing after 2025. The current deployment rate isn't fast enough to fulfill the vacuum.
Flight 1s are being upgraded, not retired and we planned to have dozens more in Flight IIIs in early 2020s. About 40 more or so. The Ohio class are being replaced by the next gen submarine so no problems with that. And the LA class are being replaced by the Virginia class boats, each of them more advanced and more capable. In fact, the first 4 Ohios are SSGNs, but will be replaced by Virginias that will be equipped with 40 cruise missiles thanks to the new payload module.