Fair question 
The answer is this: Assuming new accounts choose an initial rank closer to their true strength than 1500, then this will have reduced impact on their opponents’ ratings compared to starting them at 1500. Less impact on opponents’ ratings = improved rating system stability.
But how does that compare to their true strength? Maybe most new accounts are genuine beginners.
That plot does not show that the average true strength of new accounts is close to 1500. Of course the average rating of new accounts is close to 1500, since we start them at 1500. Those plots show the peak true strength is at more like 17-18 kyu (although it’s a bit hard to see - would be nice to see the histograms after X number of games each on separate plots so the scale is adjusted and the peaks can be seen more clearly). This is not really comparable to the present day as the system has changed significantly.
Also, the plots further down in that topic:
show the 50th percentile is around 16 kyu for September and 17-18 kyu for last year. So not 1500 = 6 kyu.
As @Vsotvep says, do not do this.
While this may be possible in theory in an extreme example, in practice it won’t work like this. While there may be a small downward drift due to an overall average of under-declared rank (e.g. true strength = 16 kyu, declares are 18 kyu), the effect will be small due to some cancellation with over-declared initial ranks (so the net is only slightly in favour of under-declared), and the fact that the under-declaration is only slight (a couple of ranks). Plus, there is still anchoring against other servers / associations, just with an offset. So we may drift to a point where X rank OGS ~ (X + 2) rank from other servers / associations. But that is OK. They’re all slightly different anyway (Rank - worldwide comparison at Sensei's Library) so these differences will also cancel out to some extent.
In the extreme scenario of every new account choosing 30 kyu, then in theory this would happen. But that’s just never going to happen in reality. We need to trust that most people will try to choose the correct rank. We can encourage this with suitable guidance information. Worst case scenario is that there is a benefit for those who choose the correct initial rank in the short term, and in the long term we end up no worse than we are now. But like I say, that is the extreme worst case. Reality will be much better than that.
This is covered by the options suggested previously (emphasis added):
and this would just revert to current behaviour, i.e. stick them in at 1500.
Would it? Where’s the evidence of that?
Would they? We cannot know either way. That’s just speculation.
That’s just anecdotal evidence. You would choose lower bound, I would chose best guess of my true rank, next person might choose upper bound. Everyone’s different so these effects will approximately cancel out on average.
As @Vsotvep has said: