Maintaining projects in assembler requires much less heroics than developing them. Significant part of Soviet computer program, shortly before (Minsk 32), and after they switched to cloning IBM mainframes (ES EVM), was maintained in assembler only, once they lost access to the original PL/S source code of the OS and other system products. These were teams of many people and the activity lasted long enough time to fit your criterium. They certainly outlived the originators by several dozen years.

Maintenance involved fixing bugs (including in the OS kernel), localizing software to a different language (sounds inoffensive ... when you have source code. Much more nasty when translated sentence doesn't fit into its allocated place in the binary). And adapting to hardware difference, such as less RAM, less capable peripherals and slower CPUs.

EDIT: Minsk 32 was bespoke development but the language still was close enough to hardware to be qualified as assembly by today's standards.