Variety Is Overrated As Well?

In the last post, the discussion was about the importance and necessity of power in tennis at the amateur. Well, another point has been brought up. Is variety overrated as well?

Variety is adding in new paces and different spins to keep the opponent out of balance and block them from playing their own game/style. Variety shots may include a fast driving stroke, and maybe a drop shot, or some slices with underspin sent crosscourt, and then a nice winner down the line. However are these all necessary?
I think the answer is no. 

When you have a strategy (for example, Nadal always hits to Federer's backhand when he is looking for the weaker side) that's nice and perfect, and the main point: IT'S WORKING, then, why would you change it? You are winning points, games, sets, and matches because that opponent cannot adapt to your strategy, therefore changing your successful plan may not be the best idea.

However, if your opponent has adapted to your plan and it becomes useless or not so successful, adding random variety is not a great choice either. Thinking of another plan that will work is the best solution, in my opinion.

Do you agree that having a solid strategy is good and you shouldn't change things up if your plan is working?

What would a 'con' be for always having the same plan?