I think 20 years might be a reasonable amount of time for hardware, which requires a heavy outlay in money to produce and which does not have almost zero cost to replicate, but not for software which can generally be copied for nothing and does not cost much to produce.
Nokia probably had a gajillion GSM patents that they used to make the phone, and a bunch of other companies had to pay them money for many years to build 2g/GSM phones.
Qualcomm and a few other companies have a bunch of patent for 4g tech, so even now (when we're rolling out 5G), it's practically impossible to make a modern...ish modem without giving atleast some money to one of those companies, and 4G is "old tech" already.
One thing that has allowed 4G to expand radily is the fact that those Qualcomm patents are under a FRAND agreement in exchange for them being including in the 4G standard.
FRAND is not with out its own problems, but it is alot better than the situation with eINK or 3-d Printers where there is not FRAND agreement