Bear in mind that shoving is limited to enemies up to one size larger, so the strategy is not always available, and may require some team-play to be enabled. And you cannot effectively spam this strategy against multiple opponents without extra investments, as in their turn they will just stand up again, so it is more useful for focus-firing. On the other hand, ranged attackers might end up with disadvantage to attack this adversary, so it could make the group offence less focused in some circumstances, which is usually suboptimal.
About monsters not being designed with these rules in mind and etc., I disagree. The way I see it, most monsters don't have proficiency bonuses to skills simply because the game assumes most creatures must default to atribute checks for skills, and monsters with some skill proficiencies really do feel unique (grimlocks are proficient in athletics!). Even the heroes will have to default sometimes. For instance, heroes not built for frontline usually do not have proficiency bonus for opposed athletics checks. Surely the game skews the whole skill system in favor of the heroes. This enables them to use the skills they are proficient with, which for me is a completely favorable consequence. If Bob the sword-and-board champion is constantly faced with monsters that can already have higher atribute scores and still frequently add proficiency bonuses to their skill checks, he will see shield mastery, or even the whole shoving rules as rather niche, and might simply lock himself in the boring hit-for-damage mode, not engaging in the more tactical and fun aspects of combat.
On a side-note, shoving should be valued by the players not only as a source of advantage on attacks. It can also be used to change the positioning of enemies, and to take advantage of hazards in the battlefield, so there is a lot of tactical potential in it.
If you are worried about the fact that shield master makes this shove a bonus action, just remember that it is a feat investment. A different fighter with a polearm build would take PAM, earning a bonus-action attack, for example, which some people will perceive as superior.