I have very mixed feelings myself. On one hand Jones and Smalling had been here long enough for us to judge that they are not United's first team level. If you put these guys alongside the various Stam, Johnsen, Rio and Vidic they will stick out like a sore thumb. On the other hand they aren't as shit as many portray them to be. In my opinion they stick in that grey area between first team and utility players. My biggest issue about the two is that they are growing old and yet they still seem to make mistakes 18 year olds do. Don't take me wrong, they aren't retiring anytime soon. However at this point in time, maturity should kick in to compensate to the slow decline physical aspects of their game. There's little maturity/football brain power in those two.
If you ask me, unless Skirniar becomes available (top quality CBs are too rare to be passed on) then I believe that United should tackle other matter first. You can't really judge your CB's pair when they are partnered with wingers as full backs and the midfield has just 1 DM and is usually played alongside an attacking midfielder in a 2 men midfield. I'd say keep the pair, strengthen the full backs positions, bring in some solid quality defensive midfield pair (Jorginho and Dier would be my favourites), play Lindelof more and send Tuanzebe on loan to a decent EPL side. That would stabilise the midfield which would allow the defence and our attack to focus on what they do best.
Then in summer 2019 we should be asking the right questions. Had Smalling/Jones improved? Is Lindelof good enough for first team? If not can Tuanzebe replace them (even in a reserve player role)? Should we move Dier in CB? Should we add a new CB? That sort of questions