Plants have to compromise between water loss, in hot climates with intense sunlight, and freely exchanging gases because they do not have separate pathways for water movement and gas exchange with the atmosphere.
If C4 plants and CAM plants had not evolved? C4 is found in only ~ 2-3% of flowering plants; there are about 7,500 known C4 species. Of the C4 plants most are monocots (sedges, grasses or orchids); nearly 5,000 C4 species are from the family of grasses. The remaining 2,500 C4 species are split nearly evenly between several tropical dicot herbs or shrubs and the monocot families of sedges and epiphytic orchids that survive dry periods.
CAM is found in cacti, agaves, euphorbia, and bromeliads. CAM Bromeliads are epiphytes where water can be erratically available in the tree canopy. Some switch from C3 to CAM depending on habitat factors.
Locations with limited water and high insolation would have fewer plant species especially the grasses adapted to the niches since all would be C3. Both C4 and CAM permit photosynthesis to proceed with less futile reactions with O2 by rubisco so there is less photorespiration. C4 & CAM are more efficient processes so can survive in locations with periods of limited water.