Nothing is definitive and we dont work for the company and things could possibly change in the future, but here is how things have historically been treated from my personal experience:
Your license is good for life for the version you purchased.
Same license from 4.x has been valid for version 5.x upgrades.
Same license from 4.x/5.x has been valid for version 6.x upgrades.
There have been huge changes from version 4.x to version 6.x during its 10+ year span. The OS went through multitude of feature enhancements, from only supporting 32bit to supporting 64bit, from supporting ide to supporting sata, to supporting drives over 2tb, to supporting even higher number of data drives, to supporting dual parity, to supporting cache drive pools, to supporting virtual machines, to supporting device pass-through, to supporting docker containers, to supporting newer cpus, to supporting more cpu cores, to supporting more memory.
Thats is why most feel comfortable in saying you will likely have the license for life and will be able to upgrade to the next version.