You make many very good points! 
While I appreciate your children are young now, the majority of Disney guests meet the height requirements and your children will too, sooner than you’ll realize. There are still a lot of attractions and shows at the parks.
As an AP, and having had 3 trips in the last 6 weeks ... the majority of guests are adults. I think it's been clear for years, maybe decade that adults (over 13) are an important target demographic for Disney. They have likely done their research and know where the money comes from. They have to build for their target market. And as a stockholder I completely support that.
Also, remember that these new lands will have more than one attraction/show.
The new Monster's land will have the coaster
AND according to DPB a show that I'm sure will have no height requirements so no loss there. The ENCANTO area in AK will likely be very young family friendly which would be a plus for young families over what was there. MK Carsland is supposed to have a lower height requirement ride, and that would put MK at a plus.
DPB * a suspended ride recreating the door vault scene – plus dining and shopping and a whole new theater show.
DHS closed one movie but opened 2 new shows plus AK has a new show opening this winter.
YUP! DHS came out ahead, and AK will have the same style show with new IP which is much more young child friendly. SO both are an increase for young families, not loss.
Until we know more details about all of the attractions coming to MK, DHS and AK, we cannot say what the info about the attractions will be.
You asked if some will have height requirements and the answer to that is ‘yes’. Disney is intended for all ages, not just the very young so requires attractions of interest to the broader spectrum.
There is already plenty of attractions for younger children that could keep them busy most the day. You also have things like Character Meets, BBB, Playgrounds, Parades and Fireworks. Character Meals are an entertainment that many young families take advantage of.
Disney Parks could not survive if they were all kiddie rides.
I was going to recommend
Legoland but they have height restrictions too. I believe you'd be hard pressed to find a theme park that does not have height restrictions. They're just a normal part of theme park visits. Possibly your family is better suited for non-theme park visits until your children are taller? There are a lot of great other options out there!
THIS^ It is up to families to decide if they are ready for a Disney Park as they are, and any other park. Disney has determined what attractions bring in the guests and dollars ~ smart business builds what customers want. Look at the lines for RnR, ToT, Expedition Everest, Tron, Guardians, Big Thunder, Flight of Passage etc ... older guests bring in more money. Adults fill those parks. Disney isn't going to leave money on the table. Guests can decide if they want to spend it there.
And I'll also say that Magic Kingdom has PLENTY for young children to do and that is the park that many young families focus on. That park already has a more than enough attractions. They aren't really taking anything away, they are adding two lands and more attractions.
Lots don't go to all the parks (exactly why Frozen and sisters were put in EPCOT), and they choose more days in MK, resort time, SeaWorld, shorter trips. We are going next year with full family with young grands. DD (a former CM) plans to take them to MK & AK only, then have resort time. When older they'll add the other parks.
