Well... like I said... I still really like open world sandbox games, even though I never finish most of them, I would neve rspend those hundreds of hours on them if I did not. So I don't see not finishing the main story as all that much of a problem.
But it is still one of the negative aspects of these kinds of games, no matter what one might think about them in general.
Also... the replayability of games, for me, usually don't come down to if I have finished them befor or not though. There are games I keep coming back to which I have finished a huge amount of times over the years, and there are other games I come back to which I have so far never finished. The size of the world matters little, open world sandbox or not matters little... actually, the games story does not matter to me either (if I have seen it once, then I really don't need to see it twice, because I already know it... I am the same with movies and tv-series, I very seldom rewatch things, especially tv-series... although, some movies I do rewatch a bit more often, specificly comic book movies, because those are the movies I have always wanted to see since I found comic back in 1992.. books are different though, I can re-read my favorit books over and over again with no problem... strange really... XD ). When I come back and play games again the only thing that really matters to me is the gameplay and the gameplay-loop. Styles of gameplay and gameplay-loops which I highly enjoy wins out every day when it comes to replaying them.
That is why I have replayed/restarted Fallout Tactics A LOT more often then I ever have Fallout 1 and 2. And I rate FO1 and FO2 as MUCH better games then FOT. Still though, FO1 and FO2 have only ever goten 1 full playthrough each, and a few started tries which I never finished, over the last 15 years... where as FOT has during the same 15 years gotten a new restart every 6-18 months, I have probably only finished the game 2 or 3 times though. Because even if FOT is totally inferior story-wise to FO1 and FO2, FOT is compleatly superior gameplay-wise. I love the combat in these game, and FOT does that a lot better then FO1 and FO2 ever did.
Currently the games I most often keep coming back to though is XCOM 2. It used to be XCOM: Enemy Unknown/Within, but I dropped that one as soon as 2 came out... I enjoy the stealth aspect of XCOM 2's combat a lot more then XCOM1's, especially with the Long War 2 mod added to it. There are maybe 2 or 3 handfull or so of other games I keep coming back to as well, some almost as much as XCOM 2, and others only once every 6 months to a year or two. None of them are open world sandbox games though, or FPS/3rd person shooters for that matter (actually... one is... I do come back to Minecraft every now and then, but usually only for a day or two befor it's out of my life again for another year or two... what draws me there though is the minging and building of stuff).
And if you look at my "top X favorit games of all times" list, then one would find that you would probably have to go down somewhere outside of my top 15, or even top 25, to start finding the majority of the games I keep replaying. I guess different parts of my brain handles and get's enjoyment from different kinds of things in a game in a different way... and as such I can have a game in my top 10 which I have only ever played once, where as a game I keep replaying multiple times in a year ends up 10-20+ places further down the list. XD