...they could have introduced more thunderbolt ports and in the same time keep the old design, i didn't see anyine moaning about that design, they just wanted a faster machine with new processors
Some company might have done that; not Apple Apple's leadership has explicitly stated on numerous occasions that they aren't interesting in doing many products. The corporate objective is to do a limited number of products extremely well.
Having a large product portfolio is not a strategic or tactical objective for Apple.
Even the old design was a compromise to merge single and dual CPU workstations into one smaller portfolio offering rather than two systems (and larger portfolio )more focused on the respective subsegments. So the old design showed the same "as simple as possible, but no simpler" approach to product portfolio. If duals had been dramatically outselling singles perhaps Apple would have kept something closer to the old design. The skew was in the other direction, so they went that way.
Other system vendors have an approach of multiple categories of overlapping products to try to put as dense a filter as possible to sweep in as much of the market they can. That isn't Apple approach. They only want a subset of the market. A profitable subset that has growth potential. Apple is no boutique vendor, they do need numbers to be viable, but pure volume isn't the issue either. They need growth because an objective is to keep the stock price up. For Apple 6-10% is good enough.
Relatively, few were complaining about the old case but relatively few were buying. No way Apple would have gone 'dark' for a couple of years on a hot product.
It boils down to providing something that is a great choice for some rather than providing the most choices.
If the Mac laptop , Mini, and iMac busineses implode and the Mac Pro market goes high growth ... maybe a "box with slots" tower comes back. In the meantime, the growth trendlines are running in the opposite direction. The laptops are dominant and the iMac is the dominant desktop.
