Well, isn't a new bike its own justification?

Actually, there are several decent ones. Anything you use quite a bit will pretty quickly "pay" for itself. If you ride regularly such as every weekend, you can probably justify the cost of a bike pretty quickly. If your bike is just going to sit in the garage, it is a waste of money no matter how inexpensive it is. Since you commute, I kind of reckon that on a usage basis, you can justify virtually any bike your little heart desires.

Secondly, bikes are cheap compared with other recreational money pits. How much does golf end up costing per round? Have you ever priced a bass boat? Motorcycles are far more expensive, burn fossil fuel and give you zero exercise.

A decent road bike right now goes for around $3,000. If you ride it 50 times per year and keep it for 6 years, you are talking about $10 per ride. Of course, you have to add in the cost of accessories, clothes, tires, tubes and so on. But you are talking less than $20 for a day of cycling (which is also good for you because of the exercise). Compared to many forms of recreation, cycling is cheap, it gives you decent exercise and it is even "green". It is hard to come up with a recreation that beats it.