Yes, the basic discovery was serendipitous. But, his research was university related, and the concept was based on a paper written 30 years earlier by another researcher. So it's not like he fell over and discovered something completely unrelated to what he was working on. This kind of thing happens all the time in academic research.
If it was discovered 'accidentally' today, it would take enormous funds and time to work out how to mass produce it, trial it and have it approved. The risk to commercialise it may turn out to be unfruitful if it was found to have some serious side effects or not be as effective as other drugs already on the market. Someone with big money would have to be convinced that it was revolutionary and that it would return huge investments.
Luckily, it emerged at a time where there was no other competing drugs in that sector. It took another 15 years before a company made the effort to attempt to commercialise it for mass production (and the catalyst for that was probably WWII). Imagine they had to go through the same approvals process and costs that exist today. The barrier to entry is too high for the little guy to enter the field, so its left up to big pharma who seem to be more interested in ineffective drugs that people take forever. Not so much one off cures.
This is obviously just my opinion, but something really needs to be done if we want to encourage difficult R&D for medicines that may not be huge immediate cash cows.