The two referendums (the older non-binding "consultation", and the newer binding), along with the results of elections, and the fact the current government is pro-independence, are the best data we've got, by mere size of sample.
There was plenty of turnover, and plenty of No votes, in the referendum famous for the use of force by the Spanish "Guardia Civil" police.
There just happened to be a lot more Yes votes. Like how people voted in another pro-independence government again in the Catalan elections organized by the Spanish government after they forcibly disolved the Catalan government, just a few weeks after this referendum.
Plus there was no active voting census and people could vote multiple times, you can't seriously consider the turnout from those events as valid in any way.
https://elpais.com/ccaa/2019/07/26/catalunya/1564132750_8266...
This and all the recent catalonian parliament elections, you clearly see the split between pro independence parties and not pro independence.
The 1978 constitution is the current rule of law, it can be changed, but that needs political support in the spanish congress. If you want to simply ignore the rule of law because you don't like the constitution or "many of the people that voted for it are dead" that's your problem (sorry to be blunt, but that can't be a serious argument).