I suggest you find a few fun couples and let them convince her. You just appear to stay neutral on the subject.
just convince her that you ant some strange and she can join or not because you are going to do it anyway...
not very helpful but probably truthful, and if you have been married for 20 years!! a little truth might be good?
If you had a disaster last time around and you've waited 20 years to raise the subject, surely you know that your wife is not going to be a willing participant in this one? If you go down the road of coercion then you need to know that unless your relationship is perfect in other ways (and sometimes even if it is) then there's a huge risk that you're going to come home from work to find your dinner in the dog and your clothing on the lawn.
Why wait this long? Wouldn't the intervening years have shown you whether or not your wife was even vaguely interested in the concept of others entering your relationship?
Personally I doubt whether anything at all will convince her to try again something of which she has only bad memories, and you need to think about whether your actions are representing her interests at all. An awful lot of people approach the concept of swinging from an oblique angle because it's kind of a way to screw other people without it actually being adultery, but unless you're both on the same page it really is just adultery, but it's adultery in which you're forcing your spouse to watch, interact and face up to your wishes, and it's frequently fatal for the relationship for these reasons.
If you bring it up to her, I hope your good a tap dancing. At one point in her life, she was willing. It didn't work out. No all these years later, you want her to do it again. If the first time was so bad, she's probably going to figure you just want to have sex with another woman. She might just take you up, call a lawyer, cut 50% away from you, and let you go fuck all the women you want.