I suppose you could be correct. Seeing a link, I was hoping it would be to a source that I could consider more authoritative, but then I have to admit that nearly all my claims about the age of this model are based on assumptions. The only solid datum I have is the 1963 copyright date in the manual, which clearly indicates an early point in this model's manufacturing life—probably the beginning.
Other than that, I can observe that while mostly vacuum-tube-based, my 502A does have a small number (perhaps three or four) of transistors, and I know that once transistors came into use, they and other semiconductor-based components very quickly obsoleted most uses of vacuum tubes. From there, I get to the assumption that by the time the 1960s decade came to an end, nearly all electronics were semiconductor-based, and I thought it therefore unlikely that Tektronix would still be making vacuum-tube-based oscilloscopes. Certainly, by this point, tabletop tube-based AM/FM radios had largely given way to pocket-sized (at least for large pockets) transistor radios. I believe that even crude integrated circuits already existed by the mid 1960s.
Perhaps there was an issue of perceived quality. This was not a time when electronic technology was perceived as advancing as rapidly as it now is; where something made today is going to be considered obsolete in a year or two.
I have no doubt that by the beginning of the 1970s, the technology was ripe for making oscilloscopes that were mostly semiconductor-based, that wee smaller, lighter cheaper, and performed better than their vacuum-tube-based ancestors. But perhaps there was a wide-spread perception that handmade vacuum-tube-based oscilloscopes were somehow of higher quality than more modern mass-produced transistor-based models, and a large enough market of consumers willing to continue to pay the higher prices for that perceived quality. Being a company that has always been perceived as a manufacturer of the highest-quality of equipment, it seems very possible that Tektronix might have been happy to make oscilloscopes this way as long as that was what the highest-paying customers wanted. I know that to this day, in the audiophile culture, there is a strong base of those who are absolutely convinced that vacuum-tube-based equipment sounds better than semiconductor-based equipment. I don't know if there is any objective scientific data to back up this belief.
It now occurs to me, comparing the two oscilloscopes that I own—The 502A which represents a mature example of a long-obsolete technology, and the T202, which represents a very immature form of a much newer technology—that as I've mentioned, I much prefer, in most cases, using the 502A over the T202. By the time the 502A was designed and built, they knew how to make a very good vacuum-tube-based, all-analog oscilloscope. The T202 was at the very early edge of non-CRT-based, all-digital oscilloscopes, and it is clear that at that point, they had yet to figure out how to make a good non-CRT-based, all-digital oscilloscope. I think it can fairly be said of the T202 that in most cases, a slightly more primitive oscilloscope, based on more primitive, but more mature technology, would be preferable to it. It's not obvious to me that any similar issue would be involved in the transition from vacuum tubes to transistors, but just because no such issue is obvious to me doesn't mean that there wasn't such an issue. Perhaps there really was a time, around the late 1960s or early 1970s, where most people found the mature vacuum-tube-based oscilloscopes to be preferable to the newfangled transistor-based ones, in much the same way that I would find a CRT-based oscilloscope of the same vintage as my T202 preferable to the T202.