The difference is timing. That's important too. Right now, the Queen's cousins are still quite active. They likely wouldn't be around, or still working, in 25 years' time. Charles and Anne aren't likely to be going at the same speed then either. I expect Andrew and Edward will slow down as well as they'll be in their 80s. While we are currently experiencing a bulge of working royals, it's going to significantly slim down through attrition in 25 years to far fewer.
Yes, that argument has been made before but I don't buy it. I don't see how adding Harry and Meghan's children will help in this respect as the lowest numbers will be reached in about 20 year when Harry and Meghan's unborn child will come of age and thus be of little use.
There has been talk about slimming down the monarchy. Currently there are 15 members doing over 50 [days of] engagements a year. That is an unnecessarily large amount of which the majority do less than 100 [days of] engagements (only Charles, William, Andrew, Edward and Anne did surpass that number by November 1; the queen, Edward (Kent), Sophie, Richard and Harry might end up around 100 depending on their activities in the last 2 months of the year).
Taking a cut-off point of 85 years (it seems that most royals stay quite active until about that age; for example, the Duke of Kent at age 83 took on 92 engagements so far this year; comparable to his 10 year younger cousin the Duke of Gloucester):
- by January 2019 there are 15 full-time royals;
- by January 2029 this number will most likely drop to about 12 (still more than enough imo),
- 10 years (2039) the numbers will be at a relatively low point (if Charles is king at that point it will be 8-9 active royals; if it's William, we're talking about 7 - the Cambridge kids are in their early 20's and can take on an engagement here and there but most likely will focus on their studies);
- another 10 years (2049) later the numbers are going up again to 9-12 full-time royals (depending on whether the Cambridge kids have spouses that become active full-time royals) which should be more than sufficient;
- another 10 years down the line (so in 40 years/2059), we're still at 9-10 full time royals.
- by 2069 (50 years from now), we might get to another low point with William, Catherine, Harry and Meghan all being in their mid- to late eighties and only George and his siblings fully active; however, as king and queen William and Catherine will most likely still take on quite a large amount of engagements; and by that point George hopefully has grown children of his own supplying another generation of royals.
So, I truly don't see why Harry and Meghan's child(ren) (and spouse(s)) - who would start becoming active by 2049 at the earliest (in their late twenties) - would be needed; especially if the support of Beatrice and Eugenie is not needed now nor in the future; as they would be the ones that really could help out at the lowest point, unlike children by Harry and Meghan, but apparently that's not considered a need.
In the Queen's family, the children of the two sons nearest the throne, i.e. her 4 'senior' grandchildren in terms of the succession, are HRH and it may be that as Charles only has two children, all his grandchildren will become HRH, even if there end up being 5 or 6 of them. In any case, I'm sure that it'll be a question of all of the Sussex children getting HRH or none.
I don't think it works that way. What if William and Catherine will have another child? In that case the 4th senior child would be the child of a future king; if not, the 4th senior child would be only the grandchild of a monarch.
Formally, all grandchildren of the monarch in male-line should be HRH; however, as this wasn't practiced for Louise and James, the most logical thing would be to formalize this new rule for all in a similar position (i.e. children of a younger child of the (future) monarch) - or elevate Louise and James to their rightful position if that is considered a mistake by now.