Bonanza has been in production longer than any other aircraft and is regarded as a real legend. It's a single-engine plane that's highly appealing to inexperienced pilots. However, aircraft performance might be well above what inexperienced pilots can handle. This doctor saying aims to convey that doctors who earn much are enjoying a bonanza, but that due to a lack of competence, they are destined to crash. In summation, it is a phrase that alludes to Bonanza's performance and attractiveness.