There are many different aspects to be considered here.
1. Conte stays or not - if Conte leaves, only acceptable replacement is Allegri (to a lesser extent Sarri, but will have to be backed in the market). Inzaghi and the rest are fucking mediocre. We will guaranteed finish below Juve, Milan, Atalanta, Napoli, even Roma, if that happens. It doesn't even matter what kind of a transfer season we have, but we will finish below them with a coach like that.
Allegri on the other hand, will be able to manage with the current squad with some backup reinforcements.
2. Mercato spend - what is the target here supposed to be?
Is it net +ve of 80m? If that's the case, then we are doomed. This implies selling Lukaku, and no signings. Any signings that will come will basically be free ones, or from the loose change that we will get by selling Vecino, Mario, etc.
Is it that we have to earn 80m in the market, but can then spend between 50-100m? This implies a net spend in the range of -30m to +30m. If this is the case, then Conte can be persuaded to stay. If we absolutely have to do this, then my preferred way would be to sell Lautaro for 70-80m. He is the most replaceable of all among him, Lukaku, Hakimi, Bastoni, etc. Mind you, I am not hitting or hating on him, but selling the others would imply far more drastic changes to our tactics.
The other option then is to sell Barella/Bastoni/Skriniar. But in that case, we would need to sell more than one player and getting quality replacements; that will imply more uncertainty.
Of course another issue currently is that there are very few suitors for Lautaro. Barcelona is out of the picture. Real Madrid can be considered, Mancity as well, but that's it.
3. Wages - A drop of 20% here is extremely difficult. There are many players who are in line for a pay raise. We can offset some by getting rid of our less essential players, and replacing them with those who will earn lesser wages. Favourites to sell here are Eriksen, Vecino, Sanchez, Vidal. But we will need to replace all of them, in addition to getting some more since our depth is abysmal.
What is terrible is how can we be in such a scenario? We had a minimal mercato last year, and have won the title. Why are Milan and Juve not in a fucking crisis?
No matter what we do though, it is clear that we cannot compete with the English clubs in the longer run. Football has been heading to this for 2 decades now, and it is getting accelerated now. Chelsea have no issue spending 200mn+, and still looking for a 100mn striker. City can spend 300mn only on defenders. Manu are only linked with players who cost 100mn each. And we are dealing with scraps here. I will have to stop watching football altogether. It's anyway a waste of time.