I doubt that you will find such travel insurnace, the longest I've been able to find is 18 months.
Consider this from the insurance company's point of view:
- Over a long term the risk profile for an individual traveller will change, not least because they will get older, but also people suffer health events
- The company's evaluation of risks generally will change
- The regulatory environment will change, legal liabilities may change
The annual renewal gives an opportunity to reassess the risk and set a new premium or decline the business. The individual needs to declare any change in their risk profile and the insurance company has the opportunity to decline the business.
From the individual's point of view it's also a chance to reassess the market, there may be better options on the market.
I don't think it's that big a deal to renew one's policy each year. My insurance renews each year, I receive notification of the renewal with a reminder of the obligation to tell the insurer of any changes in my medical condition. If I take no action my insurance renews.