A handful of large animals were released decades ago and now number in the hundreds — why has inbreeding not stopped them?

The main thing you are missing is time, measured in generations rather than in years.

For a large, long-lived mammal, sexual maturity may be several years away and generation times are long. Four decades might be only four to six generations. Inbreeding depression accumulates across generations, and at that count you are still early — the effects are present and typically not yet severe enough to stop population growth.

Compare with a fruit fly, where forty years would be hundreds of generations and the outcome would be entirely different. People intuitively convert years to "a long time" when the relevant clock is generations.

So the case you are describing is not evidence that founder effects are weak. It is a population early in a process whose consequences are still ahead of it.

Two more mechanisms worth knowing, because they explain why small populations sometimes do better than the simple model predicts.

Rapid growth reduces the damage. Inbreeding effects are worst when a population stays small for a long time. A founding group that expands quickly into empty habitat spends few generations at low numbers, so less diversity is lost to drift than the founding count alone suggests.

Purging. In a small population, harmful recessive variants are exposed in the homozygous state more often, and individuals carrying them are removed. Over generations that can strip out some of the worst variants, leaving a population that is genetically impoverished but not obviously sick. It is a brutal mechanism and it is real.

Neither means the population is healthy in any general sense. Reduced diversity leaves it fragile — poorly equipped for a new disease or an environmental shift — and that fragility does not show up while conditions are good.

The disease point deserves emphasis because it is the usual way these populations eventually run into trouble.

A genetically uniform population is uniform in its immune variation too. A pathogen that gets past one individual's defences tends to get past all of them, so outbreaks in such populations can be far more severe than in diverse ones.

So the answer to "why has it not collapsed" may well be "because nothing has tested it yet".

The second factor is severe survivorship bias in which cases you hear about.

Introductions of small numbers of animals happen constantly — deliberate releases, escapes, stowaways — and the overwhelming majority fail. They fail quietly, nobody writes about them, and there is no population left to study.

The cases that become famous are precisely the ones that succeeded, and they succeeded for a combination of reasons that include luck. Reasoning from them to "founder effects are not a problem" is like reasoning about business risk from the companies that are still trading.

The conditions that let one succeed are usually: abundant food, no predators, no competitors, a climate that suits them, and enough founders to avoid the very worst of the bottleneck. That is a demanding list and most introductions do not get it.