While it is indeed correct that, as many stealth proponents point out, air defense networks are becoming more advanced, it does not mean that stealth is necessary.
For the past years, development and proliferation of IRST and long-wavelength radars has continued. QWIP IRST, like one on Eurofighter Typhoon, can detect subsonic aircraft from 90 kilometers head-on. Long-wavelength (VHF, HF) radars can detect VLO aircraft from same distance as non-LO ones. In particular, Russians are selling Nebo VHF radars.
Modern VHF radars have resolution good enough to engage intruding aircraft, and even older ones can be used to successfully guide SAMs equipped with IR seekers close enough for them to engage stealth aircraft.
Moreover, SAM networks have never proven very effective against enemy aircraft, with missile Pk regularly being below 0,5%. Thus, it is wrong to suggest that “only”stealth aircraft can survive against advanced SAMs. In fact, during Kosovo war and intervention in Bosnia, non-stealth F-16s have proven more survivable in face of SAMs than stealthy F-117s – one F-16 was shot down by SAM out of 4 500 sorties, as opposed to 1 F-117 shot down and 1 F-117 mission killed out of 1 300 sorties – both F-117 losses happened due to a single SAM battery using low frequency signals to detect F-117s and guide IR SAMs until missiles IR seeker could take over. In Gulf War I, F-117 flew only at night; neither night-flyingF-117s or two A-10 squadrons that also flew only at night suffered any casualties.
Against VHF or HF radars, both legacy and stealth aircraft will have to use same tactics and have support of same assets to get the job done, thus removing only possible advantage of stealth. While B-2 may be large enough to avoid detection against VHF radars, it is easily detected by IR sensors, and reports have surfaced that suggest it is nowhere as stealthy as USAF says. It is also too expensive and maintenance internsive (1,14 billion USD flyaway, 3,36 billion USD unit procurement, 93 400 USD per hour of flight – all values in 2012 USD). Additionally, radar it uses for low-level flying can be easily detected by enemy passive sensors, and its stealth coating is vulnerable to rain. It carries only four times more payload than F-16, and entire 21-ship B-2 fleet was able to deliver one sortie per day during Kosovo war.
As such, jammer+limited LO+airframe performance combination has been proven superior to far more expensive all-aspect LO/VLO option. That is especially true as modern QWIP IRST can detect VLO aircraft from ranges upwards of 80 kilometers, even from front. Such sensors are already mounted on aircraft such as Eurofighter Typhoon, Dassault Rafale, and possibly PAK FA, J-20 and J-10.
As far as defense against enemy fighters is concerned, non-VLO fighters are superior in that aspect too, due to better airframe performance (not compromised by stealth requirements), higher numbers, higher sortie rates, easier maintainability and larger payload of missiles.
In future, due to proliferation of two-way fighter-missile datalinks, anti-radiation missiles and passive sensors, fighter radars are likely to stay passive, with fighters using IRST and ambient EM noise to detect each other. In such environment, stealth fighters will be as detectable as next thing in the sky. If they choose to use radar, they will be giving up their position at far greater range than they can detect a legacy fighter even if they use LPI mode.
To conculde, advanced 4-th generation aircraft with all characteristics of 5-th generation aircraft with exception of radar VLO are best choice for air defense, and are also no worse than stealth aircraft for offense operations against advanced enemies.