When the 2013 Republic annual conference meets in Leicester this weekend, Queen Elizabeth II will be 87 years old; five years beyond the average life expectancy of British females. If she seems much younger, or even ageless ("glowing", other newspapers would probably call her, paddling in their customary abyss of flattery) this is surely our fault. Our inability, or unwillingness, to see her as mortal and fragile – just another elderly woman, only richer and more photographed than most – is the central principle, and insanity, of monarchy.

If you consider the needs of constitutional monarchy, Elizabeth has done well. Perhaps she lacks the imagination, or fury, to be a rotten queen. I often wonder what her sister Margaret Rose would have done in her place; screamed like Caligula, I suspect. Elizabeth has presided over the "managed decline" of the British empire with what I imagine is called dignity. Silence, which is her established personal catchphrase, has an inherent dignity, even if most of the dignity resides in our own projection. (Jan Morris, a writer whose instincts I trust absolutely, has said that without her advisers Elizabeth does not know what to say to strangers, unless, I suspect, they are of another species).

Even so, she has always managed to convey concern for the rest. When she said, through an intermediary of course, that her greatest regret was not immediately visiting Aberfan after a pit heap collapsed on to its school in 1966, her regret seemed authentic. She has managed to convey an impression of thrift and even humility, which I find ridiculous; but no one thinks Elizabeth II a fleshpot, or selfish, or mad.

When the disaster film 2012 showed a representation of her mounting an ark to escape an exploding world, I laughed, because she would not have gone. She would not have gone without Philip, and he would not have gone at all, probably because he doesn't believe the rubbish intellectuals talk – you know the rest. She also, I suspect, benefited from the growing social mobility in the central years of her reign; gaudy privilege is easier to tolerate in a democracy when spoils are more evenly divided. (Here, monarchy seems a boastful luxury to taunt other nations, rather than a domestic insult.) Whether this concern is heartfelt, I do not know, and I do not care. I have no holy covenant with monarchy. I do not worship human gods, and I do not want to be clutched in a cold, theoretical embrace in return.

But what next? I have always felt that the British attachment to monarchy is an inch deep and a mile wide; we dream in kings and queens and dreams are by nature insubstantial – they melt in light. If it is currently robust (69% said Britain would be worse off without it in 2012) that is surely due to Elizabeth's longevity and skills. But what of her successors? Whether Prince Charles is a good man, again, I do not know or care. But he does not seem dutiful or grateful, which is the least we can expect for our money and attention. He seems, in fact, spoilt, childlike and bitter. His insult to the BBC royal correspondent Nicholas Witchell in 2005 – "I hate these people" – was an insult to all who pay the licence fee. His manic interventions in politics are known, and this is dangerous; a constitutional monarch must be beyond policy, floating smilingly, like an over-medalled cloud.

His sons, meanwhile, are princes of Tatler: profligate and ignorant, they inhabit a remote Chelsea pond. Harry is known primarily for inheriting the jocular racism of his grandfather. He once called a fellow soldier "our little Paki friend"; and what great-grandson of George VI would be so stupid as to wear a Nazi uniform as fancy dress? William just seems desperately unhappy, an anxious sacrifice too befuddled by his destiny to grasp its needs or meaning. We read of £4,000-a-night hotel suites; of £250,000 dresses; of publicly owned helicopters taking detours to amuse the princes and their friends. Last Saturday the princes attended a wedding; an RAF Sea King appeared and circled the castle. (The Ministry of Defence said there was no detour.) This is not forgivable expenditure. This is the wages of unthinking entitlement and mindless greed.

Elizabeth II was a child of war; coddled, yes, but she knew, or pretended that she knew, what world she lived in, that it was different for others – and she had the responsibility of grace. Her successors do not share it.

Twitter: @TanyaGold1