@Walter Hinteler,
Generally, I think, that since societies change, constitutions can't forever mirror the ideological, political and cultural balance within society.
An amendment to the 1978 Spanish Constitution granting the right to self-determination could perhaps resolve the issue - if approved by all Spaniards.
Spain's constitution's article 155 really is a 'nuclear option' as is proved now.
Spain's Senate is expected to approve a federal takeover of Catalonia's regional government on Friday.
Polls show Catalonia's roughly 7.5 million residents are divided on the issue of independence, though the vast majority are in favour of a legal referendum to settle the issue once and for all.
I do hope that such happens.