I don't think all the unsourced answer should be deleted at this time.
However if a question contains at least one sourced/scripture based answer, then all the other unsourced answers of that question should be deleted.
There is a question:
Why is Lord Vishnu called Aravamudhan?
I have answered it and it is accepted also. But I haven't cited any sources (actually it was during that time when I first joined the site).
Now, if someone comes and answers that question citing the scriptures, then my answer there serves no purpose at all. Actually it misguides the readers at that time as it is accepted. So, my answer must be deleted.
What is the problem in letting those answers remain as they are? It's fine if there are other scripture based answer also.
It misguides the readers. There are more than 6000 visitors per day in our site and those readers are misguided by such answers. Some answer are just based on folktales, Wikipedia, quora etc.. and it is making joke of our site.
What is the proof that visitors are misguided by such answers?
I give example of this question:
Reference to the 7 chakras in Hinduism
The accepted answer is with 15 upvotes but it is not citing any sources.
So a new user Prakash K has commented there
I am wondering why is this the actual answer, since it does not even answer the first question - "where in hindu scriptures?", it just says in yoga and tantra but does not give specific about which are the actual scriptures or books. – Prakash K Dec 26 '16 at 9:52
I have also answered the same question fully based on scripture. In that answer same user Prakash K has commented as:
Thank You. This should be the marked answer. Wonderful detailed explanation with also the specific books/scriptures containing information about it. But the word Mythology does not mean History, the Britishers were shrewd in using Mythology instead of History (itihAsa) or Ancient history (purANa) for our scriptures, and it is unfortunate that we also use it. Mythology is a study of myth (I know you already know it) and these are not myth but true histories. – Prakash K Dec 26 '16 at 11:00
So, it's clear that these type of answers are just misguiding the visitors. So if a question contains at least one sourced answer then all the unsourced answer from that question should be deleted.
In this question
The story behind Pashupatinath temple in Nepal
The accepted answer is based on wikipedia, I have also answered the same question citing scripture.
Now, that accepted answer doesn't serve any purpose remaining there. Is the purpose of our site to redirect the visitors to Wikipedia? Hinduism SE is not needed to redirect the visitors to Wikipedia, Google search does it well. There are other many many examples like this.
Fate of the deleted answer:
1) If the user is active then he can edit his deleted answer to add scriptural sources to it and then again flag moderator to undelete it.
2) Other high reputation user who can view deleted answers may edit and add sources if they want.
Why all unsourced answer shouldn't be deleted?
I'm here proposing to delete only the old unsourced answers from the question which contains at least one sourced answers. I'm not proposing to delete all old unsourced answers simultaneously because:
Our site is near to Graduation phase. The answered percentage is only about 74%. If we delete all those unsourced old answers it heavily affects our data. But if we go on deleting those unsourced answers of those question which contains at least one sourced answer then it doesn't affect data of site.
Using quality banner:
For those answer which are unsourced and which also do not contain other sourced answers in that question, we can use quality banners to such answers. Quality banner may remain until that question gets answered by some sourced answer. If that answer isn't improved upto this time also then it can be deleted.
Other things needed to implement it:
There are also other things necessary for the users (specially active users) to implement this rule:
1) Active users should change their accepted answer if they get another better answer. I rarely see users changing their accepted answer in this site. One shouldn't hesitate to change accepted answer. The owner of the previously accepted answer also shouldn't feel and take it odd. In this way best answer remains accepted and quality of the site is enhanced.
2) Users / Especially active users should accept answers to their questions if they are answered properly. I'm seeing here some active users who ask questions, make edits regularly but aren't accepting answers to their own question.
3) Active users should upvote answers which show effort and which are sourced. Even if the answer is not accepted if it is upvoted it counts as answered. Thus it helps to make our site data healthy. Users should keep their own philosophy away and should see the efforts of answer and scriptural references of answer while voting.
How to Implement:
If this proposal is accepted by community it's also very easy to implement:
Flag those answers to those question which contains at least 1 sourced answer. We can flag it as "Question already contains sourced answer so there is no need of this answer."
And based on those flags moderators can delete those answers.
This is just my Proposal. I don't know how the members of Community take this proposal. I suggest to give view through votes and comments.