This is the best advice to give to new players. Resources are limited as is, and wasting them on future or unfinished projects makes it even worse.
There are certain things that need to be completed first before starting an uncertain project just because you like a particular hero.
I recently started an alt account just to see how far can you go without spending a single penny and without wasting resources and I was surprised to see that the game nowadays gives new players enough to advance at a decent rate. It’s definitely not a sprint but it’s not a crawl either. With some smart use of the limited resources you can quickly have 20 or so maxed 3* and 4* heroes that will help you much more than unfinished 5*. And once you get there, you can afford to be picky, meaning not only which 4* or 5* hero to ascend next, but also more importantly to wait until you have all, or most of the mats needed to max that hero.
I believe the biggest and most common mistake people make in this game is when they start ascending a hero because they have nothing better to do, and then dump it for the next better thing, which they will soon dump for the next one which they can’t maximize because they now lack resources.