First thing to consider here is that no one in the front office should be thinking: 'what if this goes wrong and we don't improve?', or, 'wait a minute, we need to be careful just in case Cousins leaves in free agency years from now'. Those types of questions and thoughts are not at the forefront of their decision making, nor should they be. In their mind they are making a move that is going to help this team get to where they want to go. It is easy to criticise them for this move, but they made a bold move to help this team, unfortunately it didn't work out as planned but you never expect to fail when you make a move - you expect to succeed.
Second thing to consider here is this 'rebuild' idea or 'assets'. This move was made to free up cap space to sign players they believed would make a difference and help them compete for a play off spot. If we had made the play offs we would have been sending the a draft pick in the range of #15-30 to Chicago. After that, if we continued to make the play offs, Philly wouldn't swap their draft pick for ours, and when we did come to owe them a draft pick it would have been in the range of #15-30. A lot of play off teams throw those mid to late first round picks at veteran players, or to do deals to create cap space to sign veteran free agents. So yes, we have lost a future first round pick that could hurt a potential rebuild. However, if rebuilding was on this team's mind we wouldn't have made this trade, and arguably, we would have potentially traded Cousins and Gay for draft picks and young players to develop.
That said, you are right that we could have signed players without doing this trade, but we would not have been able to construct the roster we did. Now ok it didn't work out and we didn't achieve what our front office aimed to achieve. However, if we did achieve what our front office aimed to achieve (ie play offs), we wouldn't be having this 'negative' conversation about this trade. Instead, we would be talking about this trade in a more positive manner and about what went right and wrong with our play off run, and what we can do this off season in order to go deeper into the play offs.
Was it a good deal in hindsight? Probably not. But at the same time, they took a risk and so far they have got away with it. And as long as we improve and get better, the draft pick we send to Philly may not even cross our minds if we end up being a play off team under Joerger.