Some basic aerodynamics: All aircraft that fly low will have shorter range than flying high. The question is by how much.
Physics does not change and there is always a trade-off to be made. An engine of equal technological build, built for low level vs high level will always perform better than an engine designed otherwise. Laws of physics do not change because of technology.
This is also why Rafale is not as good at high-high (high altitude, high speed) combat as Eurofighter. It had to make design compromises.
Just like how stealth 5th generation fighters did not mean that they will be "the best" at everything. You can see that the PAKFA trades stealth for better aerodynamic performance. These are basic things that people who study physics, aeronautical engineering know.
In South Asia, given the highly sensor rich environment, once you are at medium to high altitude, everyone and their uncle can see you. Unless you are a stealth fighter. So if you start vectoring towards the border, the opponent has a good idea of what to do and where to head. This is why strike aircraft fly at low level. With MiG-27s and Jaguars on the other side and Mirage ROSE on the PAF side.
Obviously when you fly low, you have lower range. But with a specialized aircraft, you have relatively longer range, and are faster on the deck.
Why? Some ballpark aerodynamics:
The higher you fly, the thinner the air. The lower you fly, the more dense the air in the atmosphere. The thicker the air, the more drag you will have. The thinner the air, you have less drag. An engine is designed to optimize for a certain density of air.
An example perhaps is when you serve soup or dal, you prefer a certain shape of spoon. When you are serving meat, you prefer a different kind of spoon. Now, if technology improves, you can improve the spoon, but a one size fits all, will not make a single spoon that is best at serving all food.
Let me try to think of a Mastan Khan example.
You can either design a Ferrari or a Landcruiser. Technology improving does not mean you have one super duper car that can both be a Ferrari and a Landcruiser. Even if it was somehow possible, it would be extremely expensive.
At lower altitudes, ceteris paribus (everything else held constant), you would need lower volume of airflow into the engine. Same engine at higher altitudes will need a bigger volume of airflow, as the air is less dense. At lower altitude, drag increases, and density is greater. To simplify, you tend to optimize with a shorter wingspan and fatter airfoil. While at higher altitude, you need longer wingspan and thinner foils to be optimal.
These are just barest of basics, there are dozens of other paramaters, each of which has to make a tradeoff choice of for the requirements, the specifications, the flight profiles.
To have a mature conversation, one has to speak within the boundaries of what they know. Just having an opinion without a solid basis doesn't do much for the conversation.