Are there downsides in accepting a very generously paying job (compared to the other offers one may have), and which his known to be highly paying / a good position ? I am not thinking in terms of the immediate benefits (of course) but rather on a longer time scale.
I know this may sound silly but may this restrict mobility afterwards ? Even if one explains he is ready for a pay-cut, may this draw red lights like "why would he quit such a good position ?"
You already identified one potential drawback - if you later apply to another job that pays less, people will wonder just why you would leave a highly paid position, unless there is an obvious reason (like your spouse getting a job somewhere far away and you needing to take what is offered there).
High paying jobs usually come with high requirements and expectations. If expectations are higher than for a low-paying job, you can more easily disappoint the people that pay you really well.
Next, when a manager needs to trim x% from his payroll, he might be more tempted to fire one expensive employee than two cheaper ones, so your high paying job may be less secure than a low paying one.
One other thing, of course, is that you can easily get used to the high salary, either in terms of lifestyle inflation (if you spend a lot) or in terms of peace of mind (if you save a lot). Either one can restrict the possible jobs you could accept later on. However, this really is a kind of first world problem. "Help, my current job pays so well that I can't find a competitive open position elsewhere!" Lots of people would love to have this kind of problem.
There are plenty of drawbacks to higher-paying jobs. They usually boil down to one thing - with greater pay comes greater responsibility.
You may be asked / required to work longer hours. (The company wants to "get their money's worth".)
You may be put in charge of specific systems or processes, and if something happens to those systems, you're the person who has to answer for any problems.
Higher-paid people are often in management. Management is an entirely separate job skill - often people who are good at their specific job have to learn how to juggle their old duties, managing the workers under them, managing projects for their team, etc.
Even if you're not managing other workers, often senior employees (who typically earn more) are leaned on to mentor junior employees, in addition to their normal duties.
Finally, as you pointed out, when you decide to switch jobs, you will usually have to explain why you're leaving a "cushy" job (not to mention possibly having to modify your standard of living to handle a smaller paycheck).
Short answer: Higher paid jobs normally will have more responsibility and potentially a greater impact on your personal life.
When you are in junior roles, you often have a little more leeway in your level of commitment to the role. Higher paid jobs will have an expectation from your employer to give a much higher commitment to the role. You may be expected to keep your skills up to date more vigorously, and put in the extra hours.
As far as having an impact on future roles, if they are perceived to be "moving upwards", you don't have to disclose your currently salary to future employers. If it's an internal move then your employer will already understand the need to move up and will likely not be a reduction in pay anyway.
Really, the idea of a higher pay is to compensate you for the time you will lose from your non-working life, and the effort you need to put in.
(Related: I posted this recently about taking a lower role when the responsibilities of a higher role impinges on your work/life balance)
