To me that doesn't seem like a good ratio.
To know for sure, you'd have to look at ratios for other similar membership groups. I mean, what is considered a "good" ratio for total historical membership versus current active membership?
What we do have to look at is our own history. At the end of
1999, there were ~5600 current members out of almost 16000 total members. In 2006, there were ~11000 current members out of around 31500 total members. So over the course of the last 17 years, the ratio of current to total seems fairly steady at around 1/3. And membership overall is growing. For every lapsed member, we're gaining more than one new member. That seems positive to me.
The ratio is never going to be 100%. Members die, get sick, injured, enfeebled, busy, lose jobs, get new jobs, move, or quit playing for thousands of other reasons that may have have nothing to do with the PDGA at all. I think we have to assume there's always going to be a certain percentage of "dead" memberships from the moment they're purchased. A good example would be memberships for people who never actually get into the game that seriously (or at all), such as memberships bought for children before they ever pick up a disc or those who do it out of support for their significant other.
Could we be growing faster and retaining members for longer? Sure. But I don't think there's anything wrong with the rate at which the organization is growing, nor is there anything wrong with the ratio of current to total membership.