I don't think there needs to be a big change in the law, the fact is that the mechanics are in place to protect people who want protection. If you have been cleaning up after somebody's mess for 30years and want some recognition in law for that, then the ready made recognition is there in the form of a marriage certificate.
If you live together for twenty years, but you have equal jobs, no kids, and you own 90% of the equity in the house, then you might not want to marry as it would disadvantage you to share assets 50/50. You would not want for there to be some middle ground where the fact that you have lived together gives the other party rights to your assets - if you wanted that, you'd get married!
Me and my DH were in this exact position, but the assets were mine not DH's. We moved in together 7 years ago into a house purchased with my money, and with only my name on the mortgage, he was essentially my tenant, although that wasn't the way I made him feel I'd like to think! After 2 years, I was changing the mortgage deal, and it was convenient at that point for me to add him on to the mortgage, so we did, and he had to be added to the deeds of the house. I still wanted to protect my £80k stake in the house, so we had a solicitor draw up a deed of trust confirming my rights to the first £80k should we split up, or if I should die, so my parents would get the assets not DP.
I very much wanted to get married, and we finally did this year. This means now that we own the property jointly, and if I die, it will pass to him, and vice versa. I love him and would hate to think of the house being sold from under his feet, so I'm glad that us marrying has confirmed those wishes, but had he not wanted to formalise the relationship, he would also not have benefitted from the financial security the arrangement has given him.
The interesting thing is that in our situation I had the financial control, but he had the emotional control. I didn't want to be the one to propose, and it was over 7 years before he proposed, so he chose when we got married basically. In most modern relationships the man has both the financial and the emotional control, leaving the woman wanting to get married because she loves him and because she doesn't want to be left in the cold financially if it all goes tits up. And a lot of those women are still waiting.