Their CGNs also been in the water for a long time w/o going anywhere, but being modernized & rearmed. Granits r still fast enough in LACM role. As was discussed earlier, they r being given 72 new 3M55 Oniks & 3M54 Klub missiles, replacing 24 Granits during their modernization. By the same token, old Delta SSBNs r not scrapped just because Boreys r being built. Later they may be used in other roles, as Oscars.They were in the water all those 30 years. Doesn't matter they didn't go on patrol. Technology is from 30 years ago too as the structure. Granits are big, slow compared to newer russian missiles and with 30 years old technology.
The US converted 4 & keeps older Ohio SSBNs to SSGNs armed with 154 LACMs & SEALs, even though LA class SSNs were being built with VL tubes & now Block V Virginia-class SSNs. https://www.wearethemighty.com/us-builds-massive-russian-sub
Tu-95s/22s & B-52s/1Bs r also decades old, but being modernized.
China flies H-6Ks based on the Tu-16 with new engines & ALCMs.
In detail, one of the reasons for the modernization of the Akula, Sierra, and Oscar-class submarines are delays in the project 885 Yasen-class SSGN program. This class of Russian attack submarines was supposed to replace older Soviet-era multi-purpose nuclear submarine models by 2020.
Yet the exorbitant costs of the submarines — estimated to be twice as much as the new Borei-class SSBNs – has so far led only to the commissioning of one out of eight SSGNs, with a further three to four vessels likely to be completed by 2020. https://thediplomat.com/2015/10/russia-to-upgrade-12-nuclear-powered-subs/