Actually pretty realistic
"that means that ship has had the same design for at least 50 years, which again seems highly unlikely."
Perhaps some familiarity with the real world would help with this kind of analysis? Real weapon systems frequently have lifespans at least this long. An obvious comparison to star destroyers would be aircraft carriers - the first Nimitz class carrier began construction 47 years ago, while the most recent was finished just 6 years ago, and all 10 remain in active service. The M1 Abrams tank was designed in the early '70s and again remains in very active service; there have been upgrades, but nothing more visually obvious than is noted in the article. Turkey just recently shot down a plane from the '60s using one from the '70s. 50 years is not unlikely at all, it's exactly what we can see right now. And of course this is just looking at modern equipment, for most of human history weapons have remained largely unchanged for centuries or even millennia.
"it’s rare to have any technology in an advanced civilisation remain static for so long"
This depends a bit on what you define as "advanced civilisation", but given any sensible view this is again nonsense. If by "advanced" you mean "as advanced as seen in Star Wars", then obviously we have never actually seen one and the word "rare" is simply nonsense. On the other hand, if you mean it along the lines of "sophisticated tool using civilisation" then it's the exact opposite of rare - for the vast majority of human history technology has been almost entirely static. For transportation we had feet, then horses, boats, carts and finally carts without the horses, and air travel. The same applies to virtually everything else - architectural periods are defined in terms of centuries, ages (ie. bronze, iron, etc.) are even longer and are defined entirely by the dominant technologies of the time. It's only in the last century or so that there has been any real pace to the advancement of technology. The old idea of wondering what someone teleported to the modern day from 1000 years in the past makes for some nice speculation, but for the vast majority of human history someone transported 1000 years into the future wouldn't even notice the difference as far as technology goes.
Any claims about rapid technological progress in the future rely on the assumption that the current rate of progress will continue. But there's no reason to assume that to be the case, and some fairly good ones to assume it won't. Moore's law cannot continue indefinitely, nanotechnology will probably do some cool things but manipulating matter on a smaller scale gets much harder, efficiency of motors, power storage density, and many other things all have hard physical limits that we must some day run up against even if we're not there yet. When dealing with space opera you have to discount any speculation about a technological singularity, which leaves us with the simple physical impossibility of technological progress past a certain point. A galaxy-wide space-faring civilisation which has existed for tens of thousands of years cannot have a rate of technological progress as quick as we currently do in the real world. By far the most likely scenarios would be one in which the basic technology has been essentially static for centuries or longer, and design changes would be based on operational needs (from the series so far we have a large-scale war followed by internal pacification and policing of an empire followed by fighting guerillas) or simply fashion.
Of course, that all assumes that anyone involved with the film has actually given any of this a moment's thought. Given that we're dealing with fantasy space opera, not hard sci-fi, that's rather unlikely. It's just a fortunate coincidence that not thinking about it at all leads to pretty much the same result as thinking about it far too much, and it's only those who try to think about it a bit but don't get all the way there who end up with a different conclusion.