First, they don't need to look closely at squat. The only viable government "investment" is large-scale public works, and they should be structured so that any return is broad, private, and generally not financializable. Otherwise you're headed straight for planned economy where humans have a miserable track record.
Even if maturing debt is merely replaced, that still happens at less than long term average market return rates. Why worry about retiring one bond for the next generation if your estate would retire a dozen? More to the point, total-debt-to-GDP seems entirely the wrong metric: total debt service to GDP is the only thing your descendants will care about, and, barring a huge reorganization of the world economy, is something we have a good handle on for the foreseeable future.