It was introduced in 2000 and fundamentally it was about buying older people's votes.
Ultimately the BBC are part of source of their own demise - the move to digital TV allowed them to innovate and move from a licensed based to a subscription based approach but they refused to take that option preferring the soft funding of the license fee and creating ever more channels.
They've got themselves in a right pickle - they justify the license fee on the basis of 'public interest broadcasting' but then spend it on stuff like Eastenders to compete with ITV (because if they're not popular the license fee becomes hard to justify)
Then of course they are largely responsible for the rise of Farage who they've put on Question Time 'more than any other politician'
We've got a TV license but hardly ever watch BBC
And BBC's own shows, like Planet Earth 2 are already on Netflix, so why would you even still need a license fee at that point?
It made sense 20 years ago when there were only 5 or so channels, now there is so much content available so quickly (even if it's not "legal") that people are realizing it's a waste of money.
Which is true.
The BBC license fee is about £12.90/month, which to provide all the BBC National and Local Radio in addition to the multiple TV channels and their online offering is fantastic value.
Their shows will be disappearing from Netflix as rights expire in order to bring the content back to BritBox which will launch in the UK soon.
Edit - corrected fee
The BBC's funding is increasingly under threat and the license fee is being used to fund a wide range of non-core and non-BBC services; mandatory logins are a hedge against several risks.
Over-75s are entitled to a free TV license; this was historically funded by government subsidy, but that subsidy will be completely phased out by 2020-21. This will create a revenue shortfall of about £0.5bn, of a total budget of around £4bn.
On the other end of the age spectrum, an increasing proportion of young people are living in TV-less households, making it far easier for them to continue watching BBC programming while evading the license fee.
The BBC is increasingly reliant on international commercial partnerships (most notably the BritBox streaming service) in order to raise revenues, but this is potentially jeopardised by the increasing use of VPN services; GeoIP walls aren't very useful if a large proportion of internet users can choose their IP location with a couple of clicks.
Mandatory logins make a lot of possible future scenarios easier. The BBC may decide to tie logins to the license fee if rates of digitally-assisted fee evasion become untenable. The license fee may be abolished altogether by a future Conservative government, necessitating a shift to a subscription model. The BBC might also decide to open up iPlayer and Sounds to an international audience on a subscription basis.
If you object to mandatory logins, I'd suggest writing to your MP to ask them to help safeguard the BBC's funding.