I don't think you are really looking at this the right way.
If all you are really concerned about is whether in not having one will restrict airspace in which you can fly then you will probably never buy one and perhaps aircraft ownership is not for you. The question should be "how much safer is it in having a transponder", to which the answer is considerable.
Our aircraft have TCAS which is a great safety tool. The number of aircraft I see on TCAS but cannot physically see is probably 20%. If I didn't have TCAS and they didn't have a transponder then quite a few contacts would go unnoticed. One day that might not happen. Mid air crashes are not impossible.
I know people will say a good scan is all you need and all TCAS does is draw you in to forget that, but I guarantee those people have never used it.