Chaser and hurdler horses are always geldings nowadays; there were one or two in past times that were raced as entire horses; so there are no stud fees from them. The insurance pay out on a gelding such as Synchronised, who had won the Cheltenham Gold Cup, will be substantial, though not as much as for a very successful flat racing colt or horse (stud fees again). If the animal is badly injured, there is no great incentive to the owner to pay for the extensive and lengthy treatment of it in the unlikely case that it can be saved without long suffering, rather than have it humanely put down. The insurers may not be willing to pay for the treatment, anyway.
It is correct that horses get severely injured in other circumstances than the National's; we've had horses in Newmarket breaking a leg on the gallops, never mind in racing anywhere. But National fences include some where the 'drop',the landing side, is of markedly greater height than the approach side (Becher's is one). These present a real danger, and should be altered more than they have been, to make the drop equal or very nearly so. And the National has an enormous field, 40, compared to other such races, which means a far greater risk of horses being brought down, as fatally happened once this year. If both those elements were removed,the injuries and fatalities should be reduced.