I'm not sure about that. I think Celtic are eminently catchable due to the natural limitations of the SPL (no £4mill plus player comes here and it's hard to entice even £2mill plus players).
However, I'd say that it is arguable that Celtic have players worth around £2mill at least in every position of the first 11 and most of the bench. That doesn't mean they paid £2mill as some developed with them and a couple were their own youths. It also doesn't mean that they'd get £2mill for them now - due to age, I'd think Gordon and Brown would be worth little. But, for us to buy a player in contract to get similar calibre, I'd say that is fair.
We are short of that in too many areas to do it 'just' for £15mill in one close season. Especially remembering that strength in depth is important over a season. And that not all signings will work.
I'd think that we need to build over a few years. Only ever buy players for the first team and fill the bench with those we upgrade from unless there is really no depth to a position.
I'd argue that, from the current squad, we should expect that Wes, Jack, Candeias and Morelos and one of Cardoso or McRorie will be starters for a few years to come. £6-8mill more on signings might be expected to get another 3-4 long term starters. And then similar the next year. Given that not all good players will stay, it may take several years to get a first 11 back to where we want to be. Though I'd hope a good manager rather than an ok one would get us there a bit sooner.
We probably both think about £15mill would do it on the park but I think it will take longer and there will be wastage on the way no matter how good the boss do £15mill on the park may be £20-25mill spent. We can't do that all in one go and some will need funded by player sales.