Your parents' view of marriage makes it sound a little like a rock tumbler. You're stuck in this little capsule spinning around and knocking the sharp edges off each other for six decades until you're finally smoothened out and blissful (or get too old to even remember each others' names).
Marriages do have huge problems, but I believe if someone has an illness that contributes to the likelihood of problems arising, they have a duty to their spouse and to their marriage to do their utmost to seek help and try to improve. If that illness was physical and meant they couldn't work, and the family ended up living under a bridge in a big cardboard box, they would do something about it. Depression affects your loved ones just as severely as a chronic physical illness would; it may have even more serious effects than something physical, because often it expresses its symptoms primarily in chaos and unhappiness in close relationships.
I suppose in your parents' perspective a lot depends on what they see as the primary aim of a married couple, what they see as the function of a marriage. My own parents would have come from a generation in Ireland where gutting it out come hell or high water was the only option, but even they acknowledged that much misery was the result, for spouses and children alike, when one person could veto suggestions from the other (e.g. give up drinking, come straight home and don't stop at the bookies on the way, go and see a doctor about feeling suicidal) and the disempowered spouse had no other options, and felt huge social pressure to just go along and get along because marriage was a Good Thing, even if you had to go on valium to make it through the average day.
My own exH thought counselling and psychiatrists were for losers and made my life truly horrible just because of his own misplaced pride and actually sheer ignorance about mental illness. He ended up jeopardising the marriage through promiscuity, temper tantrums, financial issues because of depression-related work problems -- all of which he blamed me for. He was eventually hospitalised after threatening suicide, but by that time it was too late.
I don't think depression can be lumped together with average marital troubles in which two reasonable and mature people can sort things out and move on with their life together. How much do your parents understand about mental illness? (My exFIL was a surgeon and thought psychiatrists were a bunch of quacks until three of his children eventually had their lives turned around by lithium prescriptions) With depression, a person can be unmoved by appeals to reason, inclined to blame someone else for problems, refuse to accept responsibility for their actions and words -- in short, hard to live with and very hard to persuade to go for treatment unless a lot of people get together and push, or unless some sort of crisis occurs and treatment becomes unavoidable.
My advice to you if your H ever threatens suicide or seems to be having a mental health crisis (locking himself in a room, making threats, etc.,) is to call emergency services and let the A&E sort out what's going on. I was advised that none of my ex's problems were of my making, and there was nothing I could do to help him with his problems.