If you're making the case for immobile strikers looking out of place in the modern game, fine. But I don't see what being tall has to do with it? Both the strikers in question, Ibrahimovic and Berbatov, are not performing as they might this season, and seem to be out of place in the system their clubs are operating. But these players have already proved themselves elsewhere, just perhaps the style of those sides - Inter and Spurs - suited their games more
If a team plays with genuine wingers, and plays a more direct game, and tries to get the ball into the box as early as possibly, that's always going to suit a strong, tall striker. If your wide players are more likely to come inside than run to the byline and beat their man, and try to work the ball and constantly look for a better opportunity... said player isn't going to have the same effect
Even the top teams who play this 'modern' style need a plan B, something different for the opposition to think about. The problem with Berbatov at United for me is nothing to do with the style of player he is; rather he simply hasn't been good enough to this point
Being tall is only really an issue when coupled with immobility. C. Ronaldo is tall, but he is also athletic and very quick, for example. But smaller players at least have a low center of gravity, so they tend to be much more effective at bouncing off people and squirming through gaps, even if they are not particularly quick, and they tend to be more agile which allows them to get away from opponents more easily.
I agree with you that the best teams need to be able to vary their approach, but always have under Sir Alex, anyway, even when we don't have the right players, to my eternal frustration (Wes Brown pumping balls forward to nobody, for example). That has always been one of our greatest strengths. I sometimes pull my hair out because it's hard to understand why a player does something that likely won't achieve anything, but it's clearly because it doesn't allow the opposition to settle in to any kind of rhythm or pattern of defending. It keeps them thinking and every now and then a ball that is aimless 9 times out of 10 leads to a goal.
A "plan B" is usually achieved more with a change of personnel than style, in my experience, because it's not realistic to expect a team to change their approach too much during a game. The best you can hope for is to exploit certain areas of the pitch or to predominately use certain types of pass or cross that have looked like bringing some success. And bringing on specific players can also change your approach naturally, of course.
But you don't usually spend £30m on a player for that reason, and it's clear that we didn't, which lends credence to your assertion that Berbatov has simply not performed, rather than him not suiting the team. However, Mike Phelan has said as recently as a few months ago that they were working with him and stressing that United are a team that moves the ball quickly and that he needs to better adapt to it, which supports the idea that it is more likely a mixture of Berbatov clearly not performing, but also him not really adapting to our preferred approach.
And I would argue that the two are probably intrinsically linked. Berbatov hasn't performed because he has struggled to adapt to our approach to the game — to the speed with which we move the ball, to the intensity of our play, and perhaps to the pressure that comes with a club of this stature. The net effect is that Berbatov performs poorly and the team are poorer for it. At which point you admit that it may never work, I have no idea, but most people rightly have less patience with a 29 year old International that cost £30m, than they do with a youngster who is still finding their way in the game.