Should tenured researchers be "encouraged" (forced) to retire at a certain age?
Here in Germany, professors are public servants and automatically retire when reaching retirement age. This has the equally automatic consequence that they are not paid a wage out of the university budget any more but a pension. And pensions of public servants work the way that any money they earn is subtracted from the pension.
Thus retired professors are typically (as long as they don't mess up the working atmosphere of the department) a net gain: any lecture they hold is essentially for free, any exams they take administer (they keep the ability to act as examiner) save work for someone else in the department. OTOH, they typically have to move into some small office, and can keep lab space only if they have grant/3rd party money to pay for that.