I would say that back in the day, DIY had more of a case since loudspeaker science wasn't as well understood by anyone, and if you wanted a good one, it was easier to build something comparable to what was considered high-end. In other words, it wasn't just a hobby, it was just a way to acquire a great loudspeaker. Nowadays, DIY loudspeakers seems to be more of a hobby unto itself since commercial loudspeakers have come so far. If you are dedicated, sure, you can make something that compares to current high-end loudspeakers, but that takes a huge amount of time, resources, and effort for a solitary individual. Modern loudspeaker manufacturers have measurement gear, modeling software, and prototyping equipment that are well out of reach of the common person. That isn't even talking about the experience of veteran engineers who have decades of loudspeaker design.
That being said, going DIY still has its strengths, but they are definitely more specific than they used to be. I think that as a hobby, the design stage may only worth it for people who just want to understand the science better. Otherwise, DIYers should just go with pre-existing designs.