It is the position of the American Dietetic Association that appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Source
First, by switching to a plant-based diet, you are actually bringing down protein intake to healthy levels! There is confusion about the amount of protein needed from any source. The RDA which has been supported since the 1940s suggests that we only need 8-10% of total calories from protein and current calculations confirm this. The RDA statistically represents 98% of the population, not the average. The estimated average requirement (two standard deviations below the RDA) for the amount of protein we need is even less. Plant-based protein provides around 10% of total calories which is at the high end of the daily requirement. Therefore, by eating a WFPB diet, you are getting healthy and sufficient amounts of this nutrient as stated by the ADA. A meat-based diet actually provides higher levels of protein than the RDA which can be remedied simply by changing protein sources.
Second, meat-based protein is actually less valuable for our health compared to plant-based protein! Historically the quality of the animal protein has been wrongly portrayed by not looking at the side-effects. The high biological value of meat protein refers to the fact that its amino acid content is similar to humans which does increase its efficiency of use in the body. However, this level of efficiency may not be entirely desirable since is accelerates growth rates unnecessarily and can initiate and promote cancer. It also delivers unneeded cholesterol and saturated fats which may induce many additional problems.
Third, all nutrients work in symphony with each other! There has been too much value placed on protein as a single nutrient in the diet. By eating a range of plant-based whole foods, you actually receive the full range of amino acids, complex carbohydrates, and anti-oxidants necessary for balanced health along with a wealth of vitamins and minerals which are less abundant in meat products. In fact, many vegetables contain a sufficient amount and range of amino acids individually and, if any one is lacking, the body uses its store of amino acids to build the protein. Further, combining meat and plant proteins actually inhibits the value of the plant nutrients, so even a flexitarian diet is not necessary to achieve protein requirements, although it is a good way for those wanting to become vegan to start. All nutrients working together promote great health and this should be the main focus instead of fixating on just one part of the equation.
So by dismissing plant-proteins because of perceptions that have been created over time about the value of animal-protein and the weakness of plant-protein, one could be risking health and happiness outcomes which research and disease statistics associated with meat-based diets often confirms.