in the time it takes you to level a dragon from 6 to 7, you can level an entirely new dragon from 1 to 6, and same for 5->6 and 1->5 etc etc. while a single level 7 nets a higher price, you could train 2-6x as many lower level dragons in the same time frame and make comparable if not higher profits, and/or be more helpful for dom placements.

obviously its your fodder and your time so you can do whatever you want. i can understand exclusively training to 7+ if the fodder floor is very high, if you have limited space/starting funds, or if youre storing for a battle that's weeks away. but for all other situations it is undeniably more efficient in both time and price to not train to 7