The real question is, who will score 30+ goals for us like Ronaldo does if we sell him in the summer? Morata? Dybala? Icardi? Kean? Depay? No one is capable to to that.
You don't necessarily need a 30 goal striker. We signed Ronaldo to make the difference in big games, which he has sporadically done, but mostly it's been a failed project if you look at the CL.
If you look at our scorers over the last 10 seasons:
2011-12 - Marchisio, Vucinic, Matri 10, Vidal 7, Pepe 6, Del Piero 5, Quags 4
2012-13 - Vidal 14, Vucinic, Quags 13, Matri 10, Giovinco 9, Marchisio 8, Pogba, Pirlo 5, Licht 4
2013-14 - Tevez 21, Vidal, Llorente 18, Pogba 9, Pirlo 6, Chiellini, Marchisio, Quags 4
2014-15 - Tevez 29, Morata 15, Pogba 10, Llorente 9, Vidal 8, Pereyra 6, Pirlo 5, Bonucci 4
2015-16 - Dybala 23, Mandzu 13, Morata 12, Pogba 10, Zaza 8, Khedira, Cuads 5
2016-17 - Higuain 32, Dybala 19, Mandzu 11, Pjanic 8, Bonucci, Dani Alves, Khedira 5, Chiellini 4
2017-18 - Dybala 26, Higuain 23, Mandzu 10, Khedira 9, Pjanic 7, DC 6, Cuads, Berna 5, Benatia, AS, Matuidi 4
2018-19 - Ronaldo 28, Dybala, Mandzu 10, Kean 7, Pjanic, Can 4
2019-20 - Ronaldo 37, Dybala 17, Higuain 11, MDL, Bonucci, Ramsey 4
2020-21 - Ronaldo 27, Morata 16, Chiesa 11, McKennie, Kulu 5 (so far)
You can see that we had more diverse scorers before Ronaldo arrived, and scored just as many or more goals in total. The biggest loss is goals from midfield, but Ronaldo "steals" a lot of those by taking all free-kicks and penalties.
You can make a strong argument that teams with different options for goals are harder to stop.