You don't seem to understand, stealth is not an on/off quality that you can just strip away from an aircraft.
Stealth basically just means an aircraft have very low RCS value, several orders of magnitude smaller than a normal aircraft and therefore the distance that a radar can detect it is shorter than a normal aircraft. The point of stealth is not completely invisible but rather the ability to reduce enemy detection range so that you can detect and attack them before they can do the same to you. About your argument, let me put it this way: imagine some inventor created an electromagnetic rifle called AK-XX, this rifle can shot bullet that is as destructive as a full size tank round, but it is 1 kg heavier than a normal AK-47. Your argument is basically the same as saying well: " if the enemy can make an armor that can block AK-XX bullet then it will be more useless than a normal AK-47 because it is heavier". Well, the thing you forget is that whatever armor that can block AK-XX's bullet will also block AK-47's bullet and with even greater effectiveness.
In short, if tomorrow, the enemy can make a radar that can detect F-35 from 150 km then that very same radar will detect Rafale from 400-500 km and even more. Furthermore, the thing that people almost always forget when discussing the benefit of stealth is how it synergized with jamming power required and burn through distance of jamming.
When Jamming is factored into the radar equation, the quantities of greatest interest are Jamming to signal ratio (J/S) and Burn-Through Range.”J-to-S” is the ratio of the signal strength of the jamming signal (J) to the signal strength of the target return signal (S). It is expressed as “J/S” and often measured in dB. Apart from their unique requirements of each specific jamming technique, for jamming to be effective J must exceed S by some amount, therefore, the desired result of a J/S calculation in dB is a positive number.
Burn-through range is the radar to target distance where the target return signal can first be detected through the ECM. It is usually the range where the J/S just equals the minimum J/S requirement.
Because a stealth aircraft have lower RCS value, that means the amount of power that it must transmit to maintain the required J/S ratio is lower. Take for example, F-35 has RCS of 0.001 m2 and Rafale has RCS of 0.1 m2 and Su-35 has RCS of 10 m2. That would mean, if F-35 need 10 kW of power to jam a radar then Rafale will need 1 MW to do the same thing and Su-35 will need 100 MW.
On the other hand,if they use similar type of jamming then the burn through range against F-35will be 10 times shorter than the burn-through range against Rafale, so if a radar can neutralize F-35's jamming ability from 100 km then that very same radar will neutralize Rafale jamming from 1000 km.