The first line of the Sipser book for the Chapter: "Complexity of time," says that:
Even when a problem is decidable and, therefore, computationally solvable in principle, it may not be solvable in practice if the solution requires an excessive amount of time or memory.
My question is, yes, we have limited space and time, so the above statement makes the entire P and PSPACE concept vague, right? If so, what are these problems? Are such huge contributions acceptable in practice?