theres the well known inherent problem with the graphite at UK AGR reactors which could be very bad (can crack or misshape in such a way that the control rods or fuel rods cant be moved), not to mention the boiler cracking at the weldseams, they only mitigated this at some sites because they all are slightly different in design, they basically ignored it in the ones which didnt yet have it for decades ,the regulator ended up finding exactly that lessons learnt on older reactors were not being applied to newer ones which had the same problems inherent to them

Chernobyl lacked a containment and there are still reactors of the same type running without containment in Russia.

I think the issue comes with unknown unknowns. Before Fukushima someone might have said the same thing you just have, but a new disaster still came along and caused a lot of issues. I am still bullish on nuclear, but I think waving away concerns might do more harm than good.

The claim that disasters happen to older plants is not refuted by the observation that lessons learned are applied to the whole fleet.

One might object that there is selection bias in the original claim, due to the slowdown in construction of recent plants, but that is a separate issue. A more thorough investigation of the causes of all events leading to a significant degradation of safety margins would be needed to determine whether and how older designs are inherently more risky and whether that risk can be adequately mitigated given the constraints imposed by their design.

The fact that, prior to Chernobyl, there were several foreshadowing incidents with RBMKs which should have raised serious concerns, suggests that 'lessons learned' isn't much of a reason to be satisfied with the status quo.

Fukushima. It was a Gen 1 plant which already has the issue that a thermal runaway is possible. There were other examples of this happening like TMI. The backup for Fukushima was onsite generators which were flooded and ultimately failed causing the meltdown.

The safety lessons we learned from all gen 1 reactors was to apply passive shutdown mechanism where if input power fails fission ultimately stops. That's not something that can be applied across the fleet because it requires more infrastructure and an almost complete redesign of the reactor's setup. Which is why these early reactors all have a potential risk of thermal runaway.

Edit: It looks like all gen Is have been decommissioned as of 2015, which is great. But we really should now be talking about decommissioning gen IIs and leaping forward to Gen IVs.